Lab: Test Strips, Base Exposure and the Print's Black Point

A base exposure is the number every other number in a printing map is measured from. It is also, in most darkrooms, the number arrived at by looking at four bands of a test strip and choosing the one that looks least wrong. That produces a print. What it does not produce is a number you can write down, carry to a different enlargement three weeks later, and recover the same print from — which is what this part claims a fine print is.

This session replaces the choice with a measurement: exposures in equal fractions of a stop rather than equal seconds, a ladder anchored on a physical endpoint you can point at, a reading made dry and corrected for the one systematic error you measured in Part XVIII, and a result recorded with every setting it depends on. Two hours, ten sheets and one negative.

Purpose

To establish, for one negative on one paper with one developer at one fixed development, a reproducible base exposure: found by an f-stop ladder, anchored either on the paper’s maximum black through the negative’s clear rebate or on the important highlight, corrected for the measured dry-down of that chain, confirmed on a whole sheet judged dry, and written onto the printing map with the aperture, magnification, filter, lamp state, developer, time, temperature and agitation that make it mean anything.

It also produces one object you keep for years rather than for an evening: a ring-around, a nine-print grid a third of a stop and half a grade apart in each direction, which turns every later judgement from “is this too dark?” into “which of these nine is it nearest?”

Learning objectives

By the end of this session you will be able to:

• Lay a strip so that it crosses the tones the print will be judged on, and say why a strip across a convenient corner is a strip you have to make again.
• Generate an exposure ladder in any division of a stop, and show that both manufacturers’ published beginner strips are geometric ladders written out in seconds.
• Find the shortest exposure at which the negative’s clear rebate reaches maximum black, and say what that anchor fixes and what it leaves undecided.
• Convert the density difference between the rebate and your chosen black point into the extra exposure it needs, and state what the conversion assumes.
• Anchor on the highlight instead, giving the reason from the shape of the paper’s curve rather than as a preference.
• Apply your own dry-down figure as a stated fraction of a stop, and refuse to carry it to a different chain.
• Record a base exposure so that a stranger could reproduce it, and say what one more strip costs.

Prerequisites

• F-stop timing and the mathematics of print exposure, which owns the arithmetic. This page uses f-stop timing and derives none of it again.
• Maximum black, base white and dry-down, where your three numbers come from. Without them the session still runs and every result carries an unknown offset the page will make you write down.
• Reading the negative and making the work print, which chose the paper, the starting filtration, the black point and the white point, and opened the map this session fills in.
• Commissioning the enlarger: a base exposure measured on a machine whose illumination was never mapped is a measurement of its centre and a guess about its corners.
• Paper sensitometry, for the paper curve, the ISO range figure and reflection density.

Safety classification

Level A throughout. Three baths at tray dilution, at or near room temperature, nothing heated and nothing mixed from powder: the developer arrives at the bench in a bottle.

• The developer is alkaline, and if it is D-72 or any conventional metol-hydroquinone paper developer it contains metol and hydroquinone, both skin sensitisers and both classified as eye hazards. Gloves and eye protection whenever a solution is handled; prints lifted with tongs. The stop is a dilute weak acid and the fixer a non-hardening ammonium thiosulfate bath at working dilution.
• The enlarger is a mains appliance standing in a room with open trays of liquid. That is why the wet side and the dry side are separated, and it is the hazard a chemistry-focused reader is most likely to walk past. ILFORD’s beginner sheet asks for the same division, with a physical divide or a splash barrier between the two.
• The dark is a mechanical hazard. Two hours of carrying wet strips to a light and back with the room lights off. Clear the floor with the lights on and walk the route once.

What is not a hazard here, and why. There is no dust exposure: nothing is weighed and no jar of powder is opened, so the weighing argument that raises the developer-mixing lab to a Level B step is absent because the operation that created it is absent. There is no sulfur dioxide route, because the acid stop and the sulfite-bearing developer never meet: separate trays, separate tongs, separate waste containers. There is no concentrated silver anywhere — it is locked in hardened gelatin on the sheets or dissolved in the fixer at the low concentrations of an amateur bath. What those same substances are like as dry powders, or as a silver nitrate solution, is a different assessment entirely.

Hazards

Hazard	Where it arises	Control
Skin sensitisation from metol and hydroquinone	Developer tray; wet strips handled by hand	Nitrile gloves; tongs; a contaminated glove changed rather than rinsed
Eye irritation from an alkaline splash	Pouring the working solution; rocking a full dish	Eye protection from first pour to last rinse; rinse a splash at once and keep rinsing
Mains equipment beside open liquid	Enlarger, timer and safelight, in a room with four trays	Wet and dry sides physically separated; hands dried before touching a switch
Slips and knocks in near darkness	Carrying wet strips to a viewing light for two hours	Floor cleared with the lights on; one fixed route walked in advance
Blade injury at the guillotine or knife	Cutting a box of paper down to strips	Cut in the light, before the session; guard in place; the free hand behind the blade line
Aquatic toxicity of every spent bath	Clean-up	Collect separately, never a drain; see Disposal considerations

Required PPE

Nitrile gloves for the whole wet half of the session, changed when contaminated inside. Eye protection with side shields whenever a solution is poured or a dish rocked. One pair of tongs per bath, which ILFORD ask for explicitly: a trace of fixer or stop carried into the developer gives inconsistent results or, at worst, completely blank prints. No respiratory protection is among the controls here, and the reason is specific rather than reassuring — nothing is a powder and nothing is heated, so the airborne route that justifies a mask on the mixing page does not exist on this one.

Ventilation

An openable window or an extractor giving a through draught, run through the ventilation check before the lights go off. The airflow here is not a vapour control: no bath is heated, no solvent is present, and the most volatile thing on the bench is water. What it does is keep a closed room comfortable for two hours, which is what stops a session ending early with the last number guessed rather than measured.

Materials

Item	Quantity	Note
Variable-contrast paper, 8 × 10 in, the surface and base the final print will use	10 sheets	Two cut into strips, two whole, five halved for the ring-around, one spare
The negative, with its clear rebate accessible	1	A filed-out or glass carrier, or an offcut of the same film’s rebate, so that clear base is somewhere in the projected frame
Developer, at the dilution your source gives	1 L working	D-72 at 1+4 from the Part XVIII mixing lab, or a bought concentrate at the maker’s dilution — but one of them, all evening
Stop bath concentrate	50 mL	1+19, 1 L working
Rapid fixer concentrate	200 mL	1+4, 1 L working, non-hardening
Wetting agent	5 mL	1+200 final rinse; ILFORD state it aids even and rapid drying
Opaque card, larger than the paper	2	For masking bands; matt black on the side facing the paper
Soft pencil	1	Every strip marked on the back before it goes near a solution

Cutting the box down. Kodak define a test exposure strip as one or two inches wide, cut from a larger sheet, and say why they teach strips rather than full test prints: it is more economical. An 8 × 10 in sheet guillotined into four 2 × 10 in strips gives four for the price of one, which is the difference between a session in which you make another strip because you are not sure and one in which you do not. Cut in the light, before the session, under the dim-light handling procedure.

Chemicals

Nothing is mixed from powder on this page; the working baths are made from stock or concentrate. The quantities below are for the D-72 route, at 1+4, at 1 L of working solution, and they are what is in the tray rather than what you weigh.

Chemical	Quantity	Form
Metol	0.62 g	In solution, as the primary developing agent of 200 mL of D-72 stock
Hydroquinone	2.44 g	In solution, as the contrast agent of that stock
Sodium sulfite	9.0 g	In solution, anhydrous, as its preservative
Sodium carbonate	13.5 g	In solution, anhydrous, as its alkali
Potassium bromide	0.38 g	In solution, as its restrainer
Citric acid	50 mL of concentrate	Stop bath at 1+19, 1 L working
Ammonium thiosulfate	200 mL of concentrate	Rapid fixer at 1+4, 1 L working, non-hardening

If you use a bought paper developer, its composition is not published and the table above does not describe it: use the maker’s dilution and time, record the product and batch, and note on the map that the developer is a proprietary formulation.

Equipment

Enlarger, aligned and with its illumination mapped, from the commissioning experiment. Timer capable of a geometric series, from Part XVII; a plain timer also works, and the Alternative route says how. Easel. Filter set or colour head. Fog-tested safelight. Four dishes one size larger than the paper, one pair of tongs each. Thermometer. Guillotine or steel rule and knife. Drying rack. A viewing light of the kind and at the distance you will judge finished prints by, the same one every time — a base exposure is partly a statement about the light you looked at it in. The reflection head of the Part XV densitometer if you have one; the session does not require it.

Estimated cost

Band ££. No capital is bought here: the enlarger, timer and dishes belong to Parts XVI and XVII and the developer to Part XVIII. The band records that the session is consumed rather than kept — ten sheets and three tray-fulls, repeated for every new negative. The planner holds the dated figures and the gaps.

Estimated consumables cost

Consumed	This session	Sourced price	Cost this session
RC paper, 8 × 10 in, variable contrast	10 sheets	£33.50 for 25 sheets to £96.18 for 100	£9.60 to £13.40
Fibre-base paper, if that is the paper you are working on	the same 10 sheets	£47.04 for 25 sheets to £150.28 for 100	£15.03 to £18.82
Paper developer concentrate or stock	200 mL of D-72 stock, or 100 mL of a bought concentrate for 1 L at 1+9	£10.52 per 500 mL to £20.03 per 1 L for the bought route	£2.00 to £2.10
Stop bath concentrate	50 mL, for 1 L at 1+19	£10.66 to £12.18 per 500 mL	£1.07 to £1.22
Rapid fixer concentrate	200 mL, for 1 L at 1+4	£21.05 to £25.98 per 1 L	£4.21 to £5.20
Wetting agent for the final rinse	5 mL, for 1 L at 1+200	£28.70 per 1 L	about £0.14
Opaque masking card	not consumed	Household material; the price file holds no listing	—

The rows carrying numbers total about £17 to £22 on the resin-coated route, a floor rather than a total and wrong in one direction: high, because the stop and the fixer are charged whole to one evening while ILFORD state both can be made up and used for more than one session. The fibre row is an alternative to the RC row rather than an addition to it, and working the session on fibre puts the paper line at £15.03 to £18.82 instead. Equipment is not in the table, because an enlarger is not consumed.

The number that should govern behaviour at the easel is none of the above. It is the marginal cost of one more strip: a quarter of a sheet, £0.24 to £0.34. A printer who hesitates over a fourth strip is hesitating over a third of a pound, and will spend it several times over on the whole sheets a wrongly-read strip produces.

Waste streams

Three, and they stay apart. Alkaline developer, about 1 L, carrying negligible silver on Kodak’s own figures and governed by its pH and aquatic classification. Acid stop, 1 L, in its own container. Silver-bearing: the spent fixer with the first wash, under the silver-bearing waste procedure. Solids — gloves, and strips you do not keep — bagged, because a processed strip carries silver in hardened gelatin. All of it sits under the disposal caveat; check your local regulations.

Alternative route

The exposing half of this session needs a room that can be made dark, and there is no substitute. ILFORD are encouraging about how little that takes: print-making needs “just a room that can be blacked out”, and no running water, because the washing stage happens in daylight. Part XVI’s reversible blackout is enough.

Without an enlarger, work at contact size in Part XVI’s frame. Almost everything here survives, and one part gets easier: a contact print includes the negative’s rebate by default, so the black anchor is in every frame without a filed-out carrier or a taped offcut. What you lose is the ability to judge at the size the final print will be. Say so on the map.

Without a timer that divides stops, do the arithmetic on paper before the lights go off and run the ladder on any clock with a second hand. That is not a compromise; it is what both manufacturers already do. Compute the ideal cumulative times, round each to a tenth of a second, then take the differences, and expose those. Part XVII derives the rule and the firmware implements it.

Without a densitometer, every judgement here survives, because each is a comparison between adjacent patches rather than an absolute reading. What you cannot do by eye is put a number on your dry-down, and Part XVIII’s own alternative route gives the paired-print method instead.

Without any darkened space at all, no part of this lab can be run, and the substitute is not a smaller version of it. It is the arithmetic: build the ladders, work the recalculation for a change of magnification and grade, convert a density difference into stops, and write a map with every field filled except the exposures. A map with those fields blank is a real document; a map with estimated exposures is worse than a blank one, because a later reader cannot tell which numbers were measured.

Preparation

Do all of this with the lights on.

1. Switch the enlarger head on now. It is the first thing you do and the one most often skipped. Part XVI’s LED head page establishes the rule — the warm-up time is when the output comes within one per cent of its settled value — and puts it on the head’s label. Switch it on with the safelight and leave it on between prints: the drift is thermal, and a switched-off head begins cooling at once.
2. Check the alignment has not moved. The two-minute version from the alignment SOP, not a re-commissioning. A column collar that grips at one height and slips at another is the classic way a certificate stops being true.
3. Set the geometry once and record it. Magnification or head height, lens, working aperture, filter. ILFORD’s default is f/8, for edge sharpness and more even illumination, with an aperture giving about ten seconds — shorter times being hard to time accurately and longer ones tedious. Kodak give the same f/8. Write all four down before anything is exposed.
4. Get clear film base into the projected frame: a filed-out carrier, a glass carrier, or the negative slid so that its rebate falls inside the easel for the anchor strip only. If your carrier masks the rebate and cannot be opened out, tape a washed and fixed offcut of the same film stock over a corner of the aperture, as the Part XVIII experiment does.
5. Fill four dishes and bring every bath to temperature: 20 °C for the developer on both ILFORD paper sheets, 18 to 24 °C for stop and fixer. The thermometer stays in the developer.
6. Fix the development time now and do not revisit it. ILFORD give one minute at 20 °C for MULTIGRADE RC papers in MULTIGRADE developer at 1+9, and a range of 1:30 to 3:00 for MULTIGRADE FB CLASSIC — a range, because that sheet also states development can be extended to six minutes without any noticeable change in contrast or fog. Kodak give one minute for POLYCONTRAST IV RC in DEKTOL. A range is not permission to vary within it: pick one time and use it for the strips, the whole sheets and the ring-around alike.
7. Fix the agitation too, and notice that you are choosing between two ILFORD documents. Both paper sheets head their processing summary intermittent agitation, while the beginner’s sheet says to rock the dish continuously and Kodak’s says to agitate by rocking gently throughout. The course has no basis for preferring one and will not invent one. Choose, record which, and keep it identical all evening — a base exposure found under one agitation and used under another is a base exposure for a different developer.
8. Cut the paper, mark the strips in pencil on the back, and only then open the working packet. A packet left open under a safelight accumulates fog for as long as it stands there, and the fog test was run against a stated exposure time, not an evening.

Procedure

Stage 1 — put the strip where the print will be judged

Kodak’s instruction is one sentence and it is the whole of this stage: place the strip so that it records a good sampling of important image tones in the negative. A strip laid across a convenient corner of sky gives a beautifully graded set of greys and tells you nothing, because none of the tones on it are tones the print will be judged on. Concretely, it has to cross both anchors — the tone the previous lesson chose as the print’s black and the tone it chose as its textured white — because everything this session measures is a relationship between those two.

Where a test strip goes, and where it teaches nothing

1. 1Strip in the sky — five bands, one tone. It measures the sky and nothing else
2. 2Strip across the anchors — the chosen black, the mid-tones and the chosen textured white, all on one strip
3. 3Anchors in opposite corners — no straight strip reaches both: use two strips, or a whole test print

The bands are drawn as equal widths for clarity. On a real strip the band widths are whatever the masking card gives you; it is the tones they cross that matter, not their size.

Two consequences, and both cost paper if ignored. If the anchors are in opposite corners, no straight strip reaches both: make two strips, or make what Kodak call a test print — a whole sheet given the ladder. Kodak say the two serve the same function and differ only in size, and that they teach the strip because it is cheaper; when cheaper stops being possible, the print is still there. And the strip must land in the same place every time. Mask the easel or mark its blades: a strip that has moved between attempts changed two variables at once.

Stage 2 — build the ladder in stops, not in seconds

Paper answers to ratios of exposure. Part XVII owns the argument and this page uses its two expressions.

s = log₂(t₂ ÷ t₁)

A change of exposure, in stops

tₖ = t₀ × 2^(k ÷ n)

The general term of a ladder with n divisions per stop

t₀ is the base time, k counts the rungs away from it — negative for lighter, positive for darker — n is the number of divisions per stop, and s is a change expressed in stops.

🕰Both manufacturers already do this; neither lets you choose the division

This is worth seeing before anyone calls f-stop timing an advanced technique.

ILFORD’s beginner sheet exposes the whole sheet for 2 seconds, then covers a quarter and gives another 2, covers a half and gives another 4, covers three-quarters and gives another 8. The bands receive 2, 4, 8 and 16 seconds, and the sheet describes the result in its own words as four strips “each one stop darker than the one next to it”. That is a whole-stop geometric ladder written out as a list of seconds.

Kodak’s sheet exposes the whole strip for 5 seconds, then covers a fifth at a time for a further 2, 3, 4 and 6 seconds. The bands receive 5, 7, 10, 14 and 20 seconds, which the sheet calls “five exposures ranging over 2 stops about ½ stop apart”. The ideal half-stop ladder from 5 s is 5, 7.07, 10, 14.14, 20: Kodak have rounded each cumulative total to the nearest second and then taken the differences — 2, 3, 4, 6 — which is exactly the rounding rule Part XVII derives and the timer firmware implements.

What neither sheet gives you is the choice of division: ILFORD’s ladder is whole stops, Kodak’s is halves, and both are fixed. The content of f-stop timing as a technique is that n becomes yours.

Choose the division from the paper’s range, not from habit. Part XVII converts a published range figure into stops — a grade 2 resin-coated paper has about three stops in it — so a third-stop step is about a ninth of everything the paper can do, roughly the smallest difference most printers reliably identify on a dry print beside its neighbour. Thirds are the default here; whole stops are for the first strip on an unknown negative, sixths for the last.

Here is the seven-band, third-stop ladder this session uses, worked from a base of 9.4 s. Stage 3 explains where the 9.4 came from.

Band	Rungs from base	Ideal cumulative	Cumulative, rounded	Give at this step
1	−3/3	4.700 s	4.7 s	4.7 s
2	−2/3	5.922 s	5.9 s	1.2 s
3	−1/3	7.461 s	7.5 s	1.6 s
4	base	9.400 s	9.4 s	1.9 s
5	+1/3	11.843 s	11.8 s	2.4 s
6	+2/3	14.921 s	14.9 s	3.1 s
7	+3/3	18.800 s	18.8 s	3.9 s

Read the last column downwards and that is the session: 4.7, then 1.2, 1.6, 1.9, 2.4, 3.1, 3.9, covering one more band before each. The additions necessarily sum to the last band’s total, and the ladder spans two stops in seven rungs, its ends exactly half and double the base.

✕The two ways an additive strip goes wrong, neither visible afterwards

The card moves the paper. The masking card is placed six times without disturbing the sheet. A strip that shifts a millimetre between bands has bands that no longer correspond to areas of the picture, and nothing about the finished strip says so. Tape it down, use a masked easel, or give each band its own exposure on its own offcut.

The increments are rounded instead of the cumulatives. Round each ideal increment to a tenth and add them, and every band inherits every earlier rounding. On this seven-band strip the error is far too small to see; on a twenty-band one it is not. Round the cumulative and take differences, so that the habit is right when it matters.

Stage 3 — anchoring on the black, through the negative’s own rebate

The paper’s black is the one end of the print’s scale that is a physical endpoint rather than a preference. Past a certain exposure the deepest tone stops getting deeper, because the supply of developable crystals in the layer runs out; that plateau is what Part XVIII measured, and it is the same quantity a modern data sheet calls Dmax and that the 1894 lecture on the principles involved in enlarging, reprinted in the Hurter and Driffield memorial volume, described as “the deepest black which the paper is capable of recording when viewed by reflected light”.

The clearest place on the negative is its rebate: the strip of film outside the frame, which received no camera exposure and carries base plus fog and nothing else. Expose the ladder so that some of it falls on the rebate, and the first band whose rebate patch is not darker than the next band’s is your minimum exposure for maximum black at tonight’s geometry.

9. Compute where to centre the ladder rather than guessing: Part XVIII found this paper’s minimum exposure for maximum black at its geometry, and Part XVII’s recalculation converts it by adding the changes in stops.

🧮Carrying Part XVIII's number forward, worked

Suppose Part XVIII found maximum black at 6.0 s, f/8, 3×, filter 2, and tonight you print the same paper at 4×, f/8, filter 2. Aperture and filter are unchanged, so both contribute 0.00; had you moved from filter 2 to filter 4 there would be a term here, and Part XVII’s contested callout is where its size is argued rather than asserted. For the enlargement, Part XVI owns the relation — exposure goes as (1 + m)², not as m² — so (1 + 4)² ÷ (1 + 3)² = 1.5625, which is log₂ 1.5625 = +0.644 stops.

t = 6.0 × 1.5625 = 9.4 s. That is the centre of the stage 2 ladder, and it is a prediction. The strip is the measurement.

10. Expose the seven-band ladder from stage 2, with the rebate in the frame, and process it at the session’s fixed time, temperature and agitation.
11. Dry it, then read it under your viewing light: the anchor is the first band whose rebate patch is indistinguishable from the band after it — “differs by less than the repeatability you quoted” with a densitometer, or “cannot be separated when the two are laid edge to edge and the join is covered” without one.
12. If the last band is still darkening, the bracket ended too early. A series that never plateaus has found the end of your bracket, not the end of the paper.

!What this anchor decides, and what it decides nothing about

It decides one thing. Below this exposure no part of the print can be a true black, because the clearest thing in the optical path is not black. That makes the rebate anchor a floor under the base exposure, and floors are useful: it is what tells you a grey print is under-exposed rather than under-developed or wrongly graded.

It decides nothing about the print. No part of your picture is as clear as the rebate, so at the anchor exactly, nothing in the picture is black. It says nothing about the highlight, the grade, where the mid-tones fall, or the local moves. It is one end of a scale, not the scale.

It is the course’s own technique, and the course says so. No ILFORD or Kodak document in this corpus describes finding a minimum exposure for maximum black through clear film base; both describe picking the best-looking band instead. The nearest description in the corpus is Mike Ware’s, for alternative printing, where the reader is told to find the exposure that produces a near maximum density through the clear film base and to check it against a step tablet — a Tier 2 source, in a different process, under ultraviolet, with a different negative material. That is evidence the method is real and published, not that a manufacturer recommends it for silver gelatin enlarging.

13. Convert the anchor into a base exposure. The extra exposure your chosen black point needs beyond the rebate is the density it stands above the rebate, expressed in stops. This is the definition of transmission density and nothing more, and the course derives it here rather than citing it:

Δstops = (D_black − D_rebate) ÷ 0.301

Extra exposure a chosen black point needs above the rebate anchor

D_black is the transmission density of the tone you chose as the print’s black, D_rebate is the density of the clear rebate — which is base plus fog — and 0.301 is log₁₀ 2, the density difference of one stop. Both come off the previous lesson’s measurement of the negative.

🧮From the rebate anchor to a base exposure, worked

Your rebate reads 0.09 and the doorway you chose as the print’s black reads 0.32. The difference is 0.23, so

Δstops = 0.23 ÷ 0.301 = 0.76 stops

and the base exposure that puts that doorway at maximum black is

t = 9.4 × 2^0.76 = 9.4 × 1.694 = 15.9 s.

On the stage 2 ladder that falls between band 6 at 14.9 s and band 7 at 18.8 s — exactly where a third-stop ladder should leave you, with a neighbourhood and a reason to make one more strip in sixths.

Two assumptions. That the enlarger transmits the negative’s densities as your densitometer measured them — not quite true, since a condenser head lengthens the effective range by scattering and flare shortens it, an argument the previous lesson owns. And that the paper answers to the ratio alone, which is Part XVII’s reciprocity caveat. Where the arithmetic and the strip disagree, the strip wins: it contains the enlarger.

Stage 4 — anchoring on the highlight instead

The other end of the scale is not symmetrical with the black, and the reason is the shape of the paper’s curve.

One paper curve, and the two ends you can anchor to

• A grade 2 variable-contrast paper, drawn to teach the shape

Show the numbers behind this plot

A paper characteristic curve plotted as reflection density against relative log exposure. Density begins at a base white of 0.06 and stays almost flat for the first two-tenths of a log unit, then climbs steeply and nearly straight from about 0.3 to about 0.7 log units, reaching 1.85, then bends over and flattens to a ceiling of 2.15 which it holds from about one log unit onwards. Two markers sit on the curve. The first, low on the toe at about 0.22 log units and a density of 0.20, is labelled as the important textured highlight, and a note beside it records that the curve here is shallow, so a small change of exposure moves this tone very little in density but the difference between a tone and no tone at all is decided within a tenth of a log unit. The second, high on the shoulder at about 0.85 log units and a density of 2.10, is labelled as the important shadow at maximum black, with a note that the curve here is flat, so exposure changes are absorbed without visible effect. A horizontal guide is drawn at a density of 2.15 marked maximum black and another at 0.06 marked base white. A shaded region along the horizontal axis from 0.20 to 0.85 log units is labelled as what this negative asks the paper for. The teaching point recorded beneath is that changing the base exposure slides that whole shaded region left or right along the axis without changing its width, so a base exposure is a choice about which end of it lands where, and the two anchors are the two ends.

Series	Relative log exposure at the paper	Reflection density
A grade 2 variable-contrast paper, drawn to teach the shape	0.00	0.06
A grade 2 variable-contrast paper, drawn to teach the shape	0.10	0.08
A grade 2 variable-contrast paper, drawn to teach the shape	0.20	0.17
A grade 2 variable-contrast paper, drawn to teach the shape	0.30	0.42
A grade 2 variable-contrast paper, drawn to teach the shape	0.40	0.78
A grade 2 variable-contrast paper, drawn to teach the shape	0.50	1.18
A grade 2 variable-contrast paper, drawn to teach the shape	0.60	1.55
A grade 2 variable-contrast paper, drawn to teach the shape	0.70	1.85
A grade 2 variable-contrast paper, drawn to teach the shape	0.80	2.05
A grade 2 variable-contrast paper, drawn to teach the shape	0.90	2.13
A grade 2 variable-contrast paper, drawn to teach the shape	1.00	2.15
A grade 2 variable-contrast paper, drawn to teach the shape	1.15	2.15
A grade 2 variable-contrast paper, drawn to teach the shape	1.30	2.15

Drawn to show the shape and the two anchors, not measured from a material. The ceiling is drawn at 2.15 because that is the figure ILFORD publish for one current resin-coated paper; the toe, the slope and the base white belong to your paper, your developer and your processing, and the shape you should work to is the one you measured. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

At the black end the curve is flat: past the plateau, extra exposure changes nothing, so a shadow placed at maximum black stays there when the exposure moves. At the highlight end the curve is shallow but not flat, and it sits beside a boundary with no gradient at all — bare paper. A textured white a third of a stop over-exposed is a grey; a third of a stop under-exposed and it is paper, and once it is paper no grade change, no local work and no development trick puts anything back. That asymmetry is the whole argument for the highlight anchor.

14. To anchor on the highlight, read the same ladder for a different thing: the band at which the important textured highlight first shows a tone distinct from the paper base. That band is your base exposure, and the shadows fall where the negative’s range puts them.
15. Read both anchors off the same strip and record both. The gap between them in stops is the measurement Part XVII turns into the negative’s range — and if the two land on the same band, the negative and the grade already fit.

📷When to anchor on the highlight, and whose opinion that is

The course’s position, stated as a position: anchor on the black when the print’s structure depends on a convincing deep tone, and on the highlight when the subject lives in the light half of the scale. Portraits, high-key work, snow, mist, skin, cloth, water — anything where the viewer’s attention is in the highlights — is anchored on the highlight, and the shadows are allowed to block up, because a blocked shadow reads as depth and a blown highlight reads as a mistake.

That is photographic convention supported by the curve shape above, not a manufacturer’s recommendation, and no sheet in this corpus states it. What the corpus holds is the same preference from the 1894 lecture quoted above: a negative of a rather too contracted than a too extended range is preferred, because it is “far better to make quite sure of getting my delicate half-tones, even at some loss of the deepest black tones”. A named authority’s stated preference in a primary document is a different kind of evidence from a measurement, and this page will not upgrade it into one.

The cost is not hidden. If the negative’s range exceeds the paper’s, placing the highlight puts the shadow that would have been your black well past maximum black, and the separation in it is gone. That is a contrast problem, and it is what the next lesson’s filter and the local-control lab’s dodging exist to answer.

Stage 5 — read it dry, and correct for dry-down

16. Do not choose an exposure from a wet strip. ILFORD say to rinse the test print and examine it under a bright light; Kodak say to examine the processed strip under room lights. Neither judges it wet, and the reason is what Part XVIII measured: a print darkens between the wet sheet and the dry one, partly because the swollen gelatin collapses and the silver ends up in a thinner layer, and partly because a bright lamp over a wet surface at close range is not room light on a wall. Only the first is in the print, and only the first is what your measurement captured.
17. Dry it by the method you will dry the final print by, and give it the same time: the drying method is part of the dry-down number, not a detail of the housekeeping. ILFORD state that resin-coated prints dry in ten to twenty minutes at room temperature and must not be glazed, ferrotyped or dried on a drum or flatbed glazer, because the polythene sticks.
18. Apply the correction as a fraction of a stop, subtracted.

t_corrected = t_chosen × 2^(−d)

Base exposure corrected for dry-down

d is your measured dry-down in stops for this paper, this developer and this drying method, and the sign is negative because a print that will darken as it dries needs less exposure than the wet sheet suggested.

🧮The whole chain, end to end — and the one number in it that is invented

No manufacturer in this course’s corpus publishes a dry-down figure for a developing-out silver gelatin paper. Part XVIII established that, refused to invent one and gave a method instead; the 0.15 stops used below is an arbitrary number chosen to make the sum work, not a recommendation, a typical value or a default.

Rebate anchor from the strip: 9.4 s. Chosen black point 0.23 in density above the rebate: +0.76 stops, giving 15.9 s. Then the dry-down term:

t = 15.9 × 2^(−0.15) = 15.9 × 0.901 = 14.3 s

Round to the timer’s resolution and that is the base exposure to test on a whole sheet. Notice the size of the last term: about a twentieth of the exposure, and the same twentieth on every print made with this chain — which is what makes it worth measuring once, and what makes ignoring it a systematic error rather than an occasional one. Substitute your own figure; if you have not measured one, write on the map that the base exposure carries an unmeasured offset in one known direction, too much exposure, and print a touch light until you have.

✦Two systematic errors of similar size that point opposite ways, and do not cancel

A strip exposed in the first minutes of a session was exposed by a cold head. Part XVI’s LED head page draws the shape: output falls as the junction warms and settles within one per cent after about nine minutes on the curve it plots, a total fall of about seven per cent from cold, which is a tenth of a stop. That curve is drawn to teach the shape and is not a measurement of any head, and the commissioning experiment’s check gives scatter rather than drift, so the number to work to is the one on your own head’s label.

The direction is what matters at the easel. A cold head is the brighter one, so a strip made in the first minutes received more light than the same commanded time will deliver twenty minutes later: read a base exposure off it and every print that follows comes out light, by up to a tenth of a stop. Dry-down pushes the other way — judging wet makes you give more exposure, and the dry print comes out dark. Two errors of similar size in opposite directions is no reason to ignore either, because they are not the same kind of thing: the warm-up error is removed by procedure, and the dry-down error cannot be removed at all, only measured.

Stage 6 — a strip is not a print

19. Print one whole sheet at the corrected base exposure, on the same paper, at the same filtration, with no local work at all. Process it identically and dry it fully.
20. Judge it dry, under your viewing light, beside the marked-up work print. Wrong by a third of a stop or more, correct it and make one more; wrong by less, stop — you are inside the resolution of the judgement you are making.

Four reasons a strip does not answer for a sheet, and the fourth is the interesting one.

• A strip has no picture on it. You have been judging seven bands against each other; a print is judged against itself, against paper white, and against what it is a picture of.
• A strip has no border, and the white margin an easel leaves is the reference the eye uses for the whites.
• A strip has probably not been dried like a print, and the surface is half of a reflection reading.
• A strip’s bands were exposed in instalments and a print is exposed once. Band 7 of an additive strip received seven separate exposures totalling 18.8 s; the print receives one continuous exposure. Whether a paper responds identically to the two is an intermittency question, and Part XVII records that no manufacturer figure for it on a developing-out paper was found in this course’s corpus. A reproducible, one-directional disagreement between strip and sheet may be exactly that — and it is data. Test it properly before believing it: one additive strip against a set of single-exposure offcuts, on the same evening.

Stage 7 — the ring-around, made once and kept

An absolute judgement — “is this too dark?” — is one the eye is bad at; a comparative one — “which of these is it nearest?” — is one the eye is very good at. The ring-around converts the first into the second, permanently.

The ring-around: nine prints, one negative, made once

1. 1Exposure axis: −1/3, base, +1/3 stop — a third of a stop is about the smallest step most printers reliably see on a dry print
2. 2Contrast axis: −½, chosen, +½ grade — half-grade filters are what ILFORD supply; twelve of them across the range
3. 3The centre print — the base exposure at the chosen grade — everything else is read against it

Made once for a paper-and-developer pair, not once per negative. Nine prints on five halved sheets is an evening's cost for a reference that lasts as long as the box does.

21. Make the nine prints from one negative — a straightforward one with a full scale, not the difficult negative of the evening — on halved sheets, stepping a third of a stop each way and half a grade each way from the base exposure and grade you have just established.
22. Process them in one session, dry them by one method, and label the back of each with its two coordinates, the paper, the developer, the development time and the date.
23. Keep them flat and out of light with the box they came from. When the box changes, the ring-around refers to a chain that no longer exists.

!Where the ring-around's two axes stop being independent

The two axes are independent only while the grades are speed matched. ILFORD’s filter leaflet states that 00 to 3.5 are speed matched, so that little or no exposure adjustment is needed between them, and that grades 4 to 5 will generally require more exposure — in practice between 0.5 and 1 stop depending on the product; their comparison sheet for the current MULTIGRADE RC DELUXE says that emulsion “no longer has a large step change in exposure between grade 3.5 and grade 4”.

So build the grid inside 00 to 3.5 where you can. Above it, one of your three grade rows is also an exposure change of an unrecorded size: either measure the correction for your box, or write on the grid which row is confounded. Part XVII sets out why three ILFORD sheets give three answers and why the box in front of you outranks all of them.

No Tier 1 or Tier 2 source in this course’s corpus describes a ring-around, under that name or any other; the search was made and came up empty. It is given as long-standing darkroom practice and as the course’s recommendation, not as a manufacturer’s method.

Expected observations

Stage 3. The rebate patches darken steeply for two or three bands and then stop. The stopping is abrupt rather than gradual, which is the practical face of a paper curve having a ceiling rather than a long shoulder. Expect the plateau within about a stop and a half of the first visible tone.

Stage 3, computed against measured. Expect a disagreement of between a sixth and half a stop, with the strip wanting the longer exposure — flare and an unaccounted filter both push that way. More than a stop is a mistake rather than a property of your enlarger, and the magnification is where to look first, since it enters as a square.

Stage 4. The highlight band and the shadow band will usually be two or three apart on a third-stop ladder. If the highlight band is above the shadow band the negative is flatter than the grade and wants a harder one; if the shadow blacks several bands before the highlight has any tone, it is asking for more range than this grade has. Kodak give the same rule from the strip’s appearance: flat or muddy calls for a higher-numbered filter, harsh and contrasty for a lower-numbered one.

Stages 6 and 7. The whole sheet will look slightly less good than the best band of the strip did: you are judging a picture rather than a swatch, and what is wrong with it is what the local-control lab exists to fix. In the finished ring-around, the third-of-a-stop steps are clearly visible and the half-grade steps much subtler than most printers expect — which is why exposure errors get blamed on contrast.

What is happening chemically

Everything here rests on one chain, established in Parts IV and XVIII and used rather than re-derived: photon → electron → latent image centre → development to metallic silver. Three of its consequences are load-bearing tonight.

The print is a map of exposure, provided development is complete. Development amplifies a latent image of a few silver atoms into a fully reduced crystal, and once every crystal carrying one has been reduced, more time adds no image density. That is why development to completion is the rule for paper, and why printing by inspection destroys the relationship this session measures: with development incomplete, density is a function of exposure and time, and a base exposure becomes a number about one evening. ILFORD say the same from the other side — prints developed for shorter times may be underdeveloped and lacking in contrast and density.

Maximum black is a ceiling set by the coating, not by the bath. There are only so many developable crystals in the layer, and once all are reduced extra exposure adds nothing: that is why the rebate patches plateau and why the plateau is a usable endpoint. What you read off it is set as much by the surface as by the silver — ILFORD publish Dmax 2.15 for the current MULTIGRADE RC DELUXE against 2.05 for the paper it replaced on the same base — so the figure belongs to a paper, not to printing.

The two systematic corrections have different physical homes. Dry-down is in the print — wet gelatin is a swollen network full of water, dry gelatin a collapsed one, and the same silver in a thinner layer absorbs more of the light entering before it returns. Warm-up drift is in the lamp, an LED’s flux falling with junction temperature. Care with one does nothing for the other.

⚗Why a base exposure is a statement about the developer as well as the lamp

The paper’s characteristic curve — the mapping from exposure to density that a base exposure picks a point on — is a property of the emulsion and the developer acting on it. Change the developer, its dilution, its temperature or its age and the curve moves: the induction period lengthens, the toe shifts, and on some paper-developer pairs the image colour moves with it.

That is why the recording list below is as long as it is, and why “MULTIGRADE developer” is not an adequate entry. It is also why the ring-around carries the developer on its back: a ring-around is a reference for a chain, and a chain has more links in it than a paper.

Data to record

Everything below goes in the notebook before you leave the room.

The chain, once. Paper: maker, product, surface code, weight, size, box code and batch. Developer: product or formula version, dilution as used, temperature, mixing date, and sheets already through the tray. Stop and fixer: dilution, temperature, and the fixer’s running total. Development: the time, to the second, and the agitation pattern in words. Drying: the method by name, and the delay between the wash and the reading. Room: temperature.

The geometry, once. Enlarger, lens, working aperture, magnification or head height with the easel size, filtration, and the head’s warm-up state. Alignment and uniformity are referred to the enlarger certificate.

Every band of every strip. Strip number, band number, commanded exposure, what you saw — not a summary, because the spread is half the result.

The three headline numbers, each with what makes it meaningful.

Number	Recorded as	Why in that form
Rebate anchor	Seconds, at the stated aperture, magnification and filter	It moves with the geometry, so it is meaningless without it
Base exposure	Seconds and as stops above the rebate anchor	The seconds change when you re-enlarge; the offset does not, so long as paper and negative are the same
Dry-down correction	A fraction of a stop with its paper, developer and drying method beside it, or the note that it is unmeasured	It does not travel between chains, and a bare figure invites somebody to move it

📓What goes onto the printing map at the end of the session

The previous lesson opened the map and left three fields blank. Fill them from the table above and increment the version number: base exposure, dry-down correction applied, and known limitations — which for this session means the agitation convention you chose between two ILFORD statements, whether the head was warmed, whether the strip and the whole sheet agreed, and whether your ring-around’s grade row crossed filter 3.5.

Local moves stays blank. It is the next lab’s, and a map that quietly acquires a burn in this session is a map whose base exposure no longer means what it says.

Analysis

Compare the computed base exposure with the measured one, and keep the difference. The calculation carried forward from Part XVIII is a prediction from geometry alone; the strip is a measurement through your enlarger, with its flare, its filter and its lamp. The difference, in stops, is a property of your machine, so tabulate it — negative, magnification, computed, measured, difference — and see whether it settles.

Express the base exposure as an offset and test that it travels. Take the same negative to a different magnification, find the rebate anchor there in three bands rather than seven, and add the recorded offset. If the print matches, the offset is doing what this page claims — carrying the negative’s contribution while the anchor carries the enlarger’s. If it does not, look first at a filter change you did not account for and at a paper from a different box.

Refuse the precision your method does not have. A third-stop ladder resolves a third of a stop, so a base exposure quoted to three significant figures is a false claim about the measurement. Quote the band, the ladder’s division and the correction separately — “band 6, third-stop ladder, less 0.15 stop dry-down” says exactly what was done. With no dry-down measurement the uncertainty is neither small nor symmetrical, and an honest blank is worth more than an invented number.

Troubleshooting

What you see	Where to look
Every band too light, or every band too dark	The bracket, not the paper. ILFORD say to open up two stops if all are too light and close down one if all are too dark. Re-centre the ladder and repeat
The rebate patches never plateau	The bracket ended too early, or the developer is weak or cold. Extend the ladder upward before doubting the paper
The first print of the evening is denser than the rest, which settle after twenty minutes	Session exposure drift: the head was cold. Apply the warm-up rule and repeat the strip
The whole sheet is reproducibly lighter than the strip band predicted	Dry-down applied twice or to the wrong chain — or the intermittency question in stage 6. Check the arithmetic before claiming the physics
The whole sheet is darker than the strip band predicted	The strip was judged wet, or the dry-down correction was not applied
Bands do not line up with the areas you meant	The strip moved under the card. Tape it, or expose single bands on separate offcuts
Blacks grey and highlights fine at every band	Empty shadows, or a flat negative
Highlights blank and shadows fine at every band	Blocked highlights, or a grade too hard for this negative
The print is muddy, cold and weak whatever the exposure	A snatched print. The control for a too-dark print is less exposure, not less development
Density varies across a band that should be even	Uneven density across a strip, or uneven enlargement
Whites are not white anywhere, including the margin	Safelight fog, paper storage fog or developer fog on paper. An open packet on the bench is the commonest of the three
Parallel marks in a dried print that read as density	Squeegee lines. Squeegee everything or nothing, identically

Clean-up

Dishes and tongs washed in the order developer, stop, fixer, rinsed between and dried face down, and each pair of tongs kept with its own dish rather than pooled. Easel wiped, masking card checked for a wet corner that will mark the next sheet, guillotine blade closed and latched. Enlarger lens capped and the filter returned to its envelope — ILFORD ask for that specifically, because marks from dust, rough handling or fingers reduce print contrast and degrade the image. Run lab closing and darkroom closing.

Storage

The strips are data and are kept: sleeved, labelled on the back in pencil with the strip number, the ladder’s division, the geometry and the date, flat and out of light with the notebook page they belong to. A strip without its label is not evidence of anything. The ring-around is kept as a reference rather than a record, with the box of paper it was made from.

The developer goes back in its bottle only if it never went into a tray. Neither ILFORD nor Kodak publishes a route by which a tray of paper developer returns to stock, and a bath topped back into a bottle is a bottle of unknown bromide. The stop and the fixer are a different case, and ILFORD say so: both can be made up and used for more than one printing session. Bottle them labelled and dated, with the running sheet count on the label.

Disposal considerations

Three streams, kept apart from the moment they leave the trays. Alkaline developer carries negligible silver on Kodak’s own figures for photoprocessing solutions, so what governs it is pH and aquatic classification. Acid stop goes in its own container and not the developer’s, because combining an acid with a sulfite-bearing alkali is how a tray becomes a source of sulfur dioxide. Spent fixer and the first wash hold dissolved silver thiosulfate complexes and belong to Part XII’s silver stream under the silver-bearing waste procedure.

Collect each separately, keep it labelled and closed, and take it to a facility that accepts it. Local regulation governs and differs by jurisdiction; check your local regulations and read the disposal caveat before deciding anything from this paragraph.

Questions

1. Your ladder is a seven-band third-stop strip centred on 12.0 s. Give the seven cumulative times rounded to a tenth of a second, and the six additions you would give after the first exposure.
2. Why is the ladder anchored on the negative’s clear rebate rather than on the darkest part of the picture, and what would be wrong with the number you got from the latter?
3. Your rebate reads 0.11 and the shadow you chose as the print’s black reads 0.41, with the rebate anchor at 7.5 s. Give the base exposure before any dry-down correction, and the two assumptions the conversion rests on.
4. Part XVIII gave you a minimum exposure for maximum black of 5.0 s at f/5.6 and 3×. Tonight you are at f/8 and 5×, same paper, same filter. Where do you centre tonight’s ladder?
5. Your test strip says 14.9 s and the whole sheet made at 14.9 s comes out repeatably a little light. Give two explanations — one a mistake, one a finding — and say how you would tell them apart.
6. A print is developed for 40 seconds instead of the session’s 60 because it “looked right”. What has that destroyed, and why is less exposure the right control for a too-dark print?

Further experiments

Measure your enlarger’s constant. Extend the Analysis table to six negatives at three magnifications. If the difference between the arithmetic and the strip is approximately constant, you have earned the right to bracket three bands instead of seven; if it varies with magnification, the variation is telling you something about flare, and you should say what.

Settle the intermittency question for your paper. Expose one additive seven-band strip and, beside it, seven offcuts each given a single continuous exposure equal to one of the seven cumulative totals — same evening, same bath, same drying — and read them against each other. Part XVII records that no manufacturer figure exists for this on a developing-out paper, so a careful measurement published with its method would be a real contribution.

Put a number on the warm-up. Make the same three-band strip at one, five, ten and thirty minutes from switch-on with nothing else changed, and find the point past which the strips stop changing. That is your head’s warm-up rule measured on your head rather than read off a drawn curve, and it belongs on the head’s label beside the rest of Part XVI’s numbers.

Recap

• A base exposure is a measurement, not a preference, and only a measurement if the development time, temperature and agitation are fixed and written down with it. Put the strip where the print will be judged: Kodak’s instruction is to place it so that it records a good sampling of important image tones, and a strip across the sky measures the sky.
• The ladder goes up in ratios. ILFORD’s 2, 4, 8, 16 and Kodak’s 5, 7, 10, 14, 20 are both geometric ladders written out in seconds, and Kodak’s increments are the differences of the rounded cumulative totals — the rule Part XVII derives. What f-stop timing adds is the choice of division.
• The rebate is the anchor because it is the clearest thing in the optical path, and the first band whose rebate patch stops darkening is the minimum exposure for maximum black at tonight’s geometry. It is a floor under the base exposure and decides nothing else. The technique is the course’s own: no manufacturer sheet in this corpus describes it, and the nearest published description is Mike Ware’s, for alternative processes under ultraviolet.
• From anchor to base exposure is one line of arithmetic: Δstops = (D_black − D_rebate) ÷ 0.301, the definition of density and nothing more. Where the arithmetic and the strip disagree, the strip wins, because it contains the enlarger.
• The two ends of the scale fail differently. A blocked shadow reads as depth; a blown highlight reads as a mistake and no later decision recovers it. That asymmetry, not a preference, is the argument for anchoring on the highlight when the subject lives in the light half of the scale.
• Read the strip dry and subtract your own dry-down, which no manufacturer publishes for a developing-out silver gelatin paper and which this page does not invent. A cold head pushes the other way by a similar amount, and the two do not cancel: one is removed by switching the enlarger on early, the other only measured.
• A strip is not a print. Confirm on a whole sheet judged dry, remembering that an additive strip’s bands were exposed in instalments and the print was not. Then record the base exposure twice: in seconds, which change when anything moves, and as stops above the rebate anchor, which do not. And make the ring-around once and keep it, because it converts an absolute judgement the eye is bad at into a comparison the eye is good at.

Check your understanding

Question 1. Kodak’s beginner sheet builds a test strip by exposing the whole strip for 5 seconds and then covering a fifth at a time for a further 2, 3, 4 and 6 seconds. What is that sequence, arithmetically?

• An arbitrary set of round numbers chosen to be easy to count
• A half-stop geometric ladder — 5, 7.07, 10, 14.14, 20 seconds — with each cumulative total rounded to the nearest second and the differences of the rounded totals given as the increments
• An equal-seconds ladder, which is why the darker bands look so similar
• A ladder in equal density steps rather than equal exposure steps

Show the answer and why

Answer: A half-stop geometric ladder — 5, 7.07, 10, 14.14, 20 seconds — with each cumulative total rounded to the nearest second and the differences of the rounded totals given as the increments

The cumulative totals are 5, 7, 10, 14 and 20, and the sheet itself calls them five exposures over two stops about half a stop apart. The ideal half-stop series from 5 seconds is 5, 7.071, 10, 14.142, 20, so Kodak have rounded each cumulative total and then taken differences — 2, 3, 4, 6 — rather than rounding the increments and adding them up. That is precisely the rule Part XVII derives for keeping rounding error from compounding along a long strip, and it means the technique called f-stop timing is not a departure from manufacturer practice but a generalisation of it: what the sheet does not give you is any way to choose a division other than the half stop it printed.

Question 2. You expose a ladder through a negative with its clear rebate in the frame, and find that the rebate patch stops getting darker at 9.4 seconds. Your rebate reads a transmission density of 0.09 and the doorway you chose as the print’s black reads 0.32. What is the base exposure that puts the doorway at maximum black?

• 9.4 s, because that is where maximum black was reached
• About 15.9 s: the doorway stands 0.23 in density above the rebate, which is 0.23 ÷ 0.301 = 0.76 stops, and 9.4 × 2^0.76 = 15.9
• About 12.0 s, since density and stops are the same quantity
• It cannot be calculated, because paper density and negative density are unrelated

Show the answer and why

Answer: About 15.9 s: the doorway stands 0.23 in density above the rebate, which is 0.23 ÷ 0.301 = 0.76 stops, and 9.4 × 2^0.76 = 15.9

A density difference is a logarithm of an opacity ratio, so dividing it by 0.301 — the logarithm of two — converts it directly into stops. The rebate is base plus fog and is the clearest thing in the optical path, so it reaches maximum black first; every part of the picture is denser and needs proportionally more exposure. The conversion assumes the enlarger transmits those densities as the densitometer measured them, which a condenser head and enlarger flare both disturb, and that the paper responds to the ratio alone. That is why the number is a prediction: where it and the strip disagree, the strip is the one that contains the enlarger.

Question 3. Why does the course anchor on the highlight rather than the black for a high-key portrait, and what kind of claim is that?

• Because ILFORD’s data sheet recommends it for portrait work
• Because the paper curve is flat at the black end, so a shadow past maximum black stays put, while at the highlight end the curve runs into bare paper, where a tone that is a third of a stop light is gone and no later decision recovers it — and the recommendation itself is photographic convention, not a manufacturer’s rule
• Because highlights need more exposure than shadows
• Because it uses less paper

Show the answer and why

Answer: Because the paper curve is flat at the black end, so a shadow past maximum black stays put, while at the highlight end the curve runs into bare paper, where a tone that is a third of a stop light is gone and no later decision recovers it — and the recommendation itself is photographic convention, not a manufacturer’s rule

The two ends of the scale fail asymmetrically, and the asymmetry is in the shape of the curve rather than in anyone’s taste: past the plateau the paper has no more density to give, so an over-exposed shadow simply stays black, whereas an under-exposed textured white becomes base white, which has no gradient at all and therefore no information to recover. The mechanism is sensitometry; the decision to prefer the highlight when the subject lives in the light half of the scale is convention. The corpus holds a related preference stated in 1894, in the lecture on the principles involved in enlarging reprinted in the Hurter and Driffield memorial volume, which prefers a slightly contracted negative range so as to be sure of the delicate half-tones even at some loss of the deepest blacks — a named authority’s stated preference, which is a different kind of evidence from a measurement.

Question 4. The first print of a session comes out denser than the four that follow it, and by about a tenth of a stop. What is the most likely cause, and how does it interact with dry-down?

• Developer exhaustion, which is worst at the start of a session
• A cold enlarger head: LED flux falls as the junction warms, so the first exposures deliver more light than the same commanded time will twenty minutes later — an error that pushes the opposite way from dry-down and does not cancel it, because one is removed by procedure and the other can only be measured
• Safelight fog, which accumulates as the session goes on
• The paper warming up in the room

Show the answer and why

Answer: A cold enlarger head: LED flux falls as the junction warms, so the first exposures deliver more light than the same commanded time will twenty minutes later — an error that pushes the opposite way from dry-down and does not cancel it, because one is removed by procedure and the other can only be measured

Exhaustion runs the other way and is a slow decline across many sheets, not a step at the first one. The signature here is a single early sheet that is denser, then a settled run, which is thermal: at constant current an LED’s flux falls as its junction temperature rises, and the head reaches equilibrium after several minutes. The fix is procedural — switch the head on with the safelight and leave it on. Dry-down is the opposite sign, because judging a wet print makes you give more exposure than the dry sheet wants. Treating the two as cancelling is a mistake: the warm-up error disappears entirely once you keep the rule, at which point the uncorrected dry-down error is on its own and undiminished.

Question 5. Why does this session insist on confirming the base exposure on a whole sheet rather than trusting the best band of the strip?

• Because strips are always processed differently from prints
• Because a strip has no picture and no border to judge whites against, is dried differently, and — the reason that is a real open question — has bands built from several separated exposures where the print gets one continuous one, and no manufacturer figure for that intermittency effect on a developing-out paper was found in this course’s corpus
• Because the strip fades before the print is made
• Because the strip is exposed at a different magnification

Show the answer and why

Answer: Because a strip has no picture and no border to judge whites against, is dried differently, and — the reason that is a real open question — has bands built from several separated exposures where the print gets one continuous one, and no manufacturer figure for that intermittency effect on a developing-out paper was found in this course’s corpus

Three of the reasons are about judgement: a swatch is compared with its neighbours while a print is compared with itself, with paper white, and with what it is a picture of, and the easel’s white margin is part of that reference. The fourth is physical and unresolved. On an additive strip the darkest band received its total in six or seven instalments; the print receives it in one. Whether a photographic paper integrates those identically is the intermittency question, and this course has not found a published figure for it on a developing-out paper. That is why a reproducible, one-directional disagreement between strip and sheet is worth measuring properly rather than shrugging at — and also why you check your arithmetic first, since applying a dry-down correction twice produces exactly the same symptom.

Question 6. You are building a ring-around on a paper at a chosen grade of 4, stepping half a grade each way. What is wrong with the grid, according to ILFORD’s own documents?

• Nothing: the twelve MULTIGRADE filters are described as evenly spaced and speed matched
• The grade axis has crossed the speed-matched boundary. ILFORD state grades 00 to 3.5 are speed matched and that 4 to 5 will generally need more exposure — in practice between 0.5 and 1 stop depending on the product — so a grade row above 3.5 is also an exposure change of an unrecorded size
• Grade 4 cannot be used for a ring-around because it has too little exposure range
• The half-grade steps become invisible above grade 3

Show the answer and why

Answer: The grade axis has crossed the speed-matched boundary. ILFORD state grades 00 to 3.5 are speed matched and that 4 to 5 will generally need more exposure — in practice between 0.5 and 1 stop depending on the product — so a grade row above 3.5 is also an exposure change of an unrecorded size

The filter leaflet does say twelve evenly spaced, speed matched grades in its opening line, and then qualifies it in the next paragraph: 00 to 3.5 are speed matched, and 4 to 5 generally require between half a stop and a stop more depending on the product. So above 3.5 the grid’s two axes are no longer independent — a row that was supposed to vary contrast alone is varying exposure too, by an amount the maker declines to fix. ILFORD’s comparison sheet for the current MULTIGRADE RC DELUXE adds that this emulsion no longer has a large step change there and needs only fine tuning, which is a third answer again. The practical response is not to pick a sheet to believe but to measure the correction on your own box, which takes twenty minutes and outranks all three.

Question 7. Why is a base exposure quoted twice — in seconds, and as stops above the rebate anchor?

• Redundancy, in case one is written down wrongly
• Because the two carry different things: the seconds contain the enlarger’s geometry and lamp and stop being true the moment anything moves, while the offset in stops is the negative’s own contribution and stays true across a change of magnification, aperture or lamp so long as the paper and the negative are the same
• Because some timers read in seconds and others in stops
• Because the seconds are for the strip and the stops are for the print

Show the answer and why

Answer: Because the two carry different things: the seconds contain the enlarger’s geometry and lamp and stop being true the moment anything moves, while the offset in stops is the negative’s own contribution and stays true across a change of magnification, aperture or lamp so long as the paper and the negative are the same

The rebate anchor is a measurement of the enlarger and the paper together at one geometry, and it moves whenever the geometry does. The offset between that anchor and the base exposure is a measurement of the negative — specifically, of how far the chosen black point stands above base plus fog — and it does not move. Separating them is what lets you re-print at a different size by re-finding the anchor in three bands and adding a number you already have, instead of running the whole ladder again. It is the same reasoning that makes a burn recorded in stops portable and a burn recorded in seconds a record of one evening.

Check my answersClear