Lab: A Step-Wedge Exposure Series

Every number in the previous four lessons came from somebody else. This session produces one strip of film that carries, in twenty-one grey rectangles, the whole response of your emulsion in your developer across ten stops of exposure — measured in a single instant, so that nothing except the light is allowed to vary along it.

It will not be a good measurement. The wedge is nominal, the enlarger is not a sensitometric illuminant, and until Part XV you have no densitometer. What it will be is yours, repeatable and honest about its own limits, which is more than any published curve can offer you about your own darkroom.

Purpose

To expose one 21-step transmission wedge by contact onto film under the enlarger, from a geometry that is measured and written down; to process that strip with an unexposed control strip in the same tank at the same time; to read the twenty-one steps by an interim method whose precision you have measured rather than assumed; to plot the first characteristic curve of your own material; and to archive the strip so that Part XV’s densitometer can read it again and tell you how good this reading was.

Learning objectives

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

• Explain why a step wedge is an intensity-scale exposure and why that matters for the curve it produces.
• Set an enlarger up as a light source of known and measured uniformity, and state the uniformity as a number in wedge steps.
• Choose a base exposure by pilot bracket so that the series spans toe to shoulder, and correct it by a stated number of steps.
• Process a strip with a control strip and record everything needed to repeat the session.
• Read densities by lux-meter densitometry, by spot meter and by visual matching, and state the resolution of each from your own measurement.
• Plot the curve, locate the criterion point and compute a contrast index from your own numbers.
• Say precisely which of your figures are relative and which, if any, are absolute — and why the answer to the second is none.

Prerequisites

• Assignment: interpreting density data, which is where you learned to do all of the arithmetic on numbers that could not bite back.
• Lab: processing your first film, for the clearing-time test, the wash and the drying, none of which is repeated here.
• Experimental design for the darkroom and Lab: preparing standard test negatives, which own the step-wedge readings, the masked patch, the notch codes and the archive. This page uses all of them and restates none.
• The course’s speed criterion and its contrast-index convention.
• The SOPs for opening and closing the laboratory, mixing from a stock and labelling a container.

Safety classification

Level A: a standard home darkroom, with one electrical hazard added by the enlarger and one mechanical hazard added by working in the dark.

The chemistry is the same chemistry, at the same dilutions and in the same quantities, as Part VI’s processing lab and Part IX’s test-negative lab: a developer diluted from a stock you mixed, a stop or a water rinse, a proprietary fixer at the maker’s dilution, and a wash. Nothing is weighed out. Two or three hundred millilitres of each, handled with gloves and eye protection, is squarely Level A.

What is not a hazard here, and why. The enlarger’s lamp is a tungsten or tungsten-halogen source behind a condenser or a diffuser, and it emits no ultraviolet worth controlling at the film plane through that glass — this is not a UV process and there is no requiresUV control on this page. The step wedge itself is a sealed article: a silver or dye image laminated in a polyester base, sold as an article rather than as a chemical, and it presents nothing to absorb, inhale or swallow so long as it is not cut, burned or chewed. The developed film is likewise an article, and the small mass of silver in a 135 mm strip is locked in a hardened gelatin layer where it is neither soluble nor bioavailable. Contrast that with the same silver as silver nitrate on the bench in Part V, where it is a corrosive, light-sensitive salt with a real handling regime; the difference is entirely the chemical form and the matrix, and being able to see that difference is a large part of what a hazard assessment is.

The mains hazard is real and is why requiresMains is set: the enlarger is a mains appliance switched repeatedly, often by a foot switch or a timer, in a room with water in it.

Hazards

Mains electricity and water in the same room. The commonest genuinely dangerous thing in a darkroom. The enlarger, the timer and the lamp are plugged into a socket protected by a residual current device; nothing wet goes near them; the wet bench and the dry bench are separate surfaces, as Kodak’s own darkroom guidance sets out, and hands are dried on a dedicated towel before touching a switch.

Skin sensitisation from the developing agents. Metol is notified as H317, may cause an allergic skin reaction, and hydroquinone carries the same statement. Diluted 1+1 and poured into a closed tank, the exposure route that remains is a splash on the hand, so gloves and a lidded vessel are the whole of the control. The reason to take it seriously on a session this small is that sensitisation is cumulative and permanent: it is the one hazard on this page where a hundred careless afternoons matter more than any single one.

Rapid fixer: eye and skin irritation, and a smell that is not the hazard. Follow the supplier’s own safety data sheet for the product you have; the course does not restate a classification for a mixture whose composition it has not read. The ammonia smell of an ammonium thiosulfate fixer is an amenity problem the ventilation deals with, not the thing that would hurt you.

Acid meeting sulfite waste. Not a hazard within this procedure if you use the water rinse, and a real one in the waste bottle if an acid stop reaches the developer container. Two containers, labelled, never combined.

Working blind. A panchromatic film has no safelight, so the loading is done in total darkness with solutions, glass and an enlarger baseboard nearby. Everything on the floor is moved before the light goes off, and every vessel is placed and rehearsed with the light on first.

Sharp edges worked by touch. A freshly cut film edge will open a finger, and so will the corner of a 3 mm glass sheet. Tape the glass edges.

Aquatic toxicity of the developer waste. Both developing agents are notified as very toxic to aquatic life with long-lasting effects. A session this small consumes almost none of them, so nearly all of what was weighed out in Part VIII is still in the bottle when you pour it away.

Required PPE

• Single-use nitrile gloves, 0.2 mm, the splash-resistant grade HSE’s COSHH essentials sheet P1 specifies for manual film development where the safety data sheet gives no more specific advice. Changed when contaminated and thrown away at the end.
• Eye protection whenever a solution is poured or a tank inverted.
• An apron or overall kept for laboratory work.
• Dry, clean hands for the film and the wedge. The Library of Congress’s handling guidance — freshly washed hands, or clean lint-free cotton or inert plastic gloves, and never a finger on the image area — is the control that protects the archive rather than you. The wedge is an instrument and a fingerprint on it is a permanent density error on every strip you make afterwards.
• A residual current device on the socket the enlarger is plugged into. This is protective equipment even though it is not worn.

Ventilation

Ventilation is not being asked to capture a vapour here; nothing on this bench evaporates appreciably at 20 °C. It is doing two other jobs, and HSE’s COSHH essentials sheet P1 sets the standard for both: general ventilation above five air changes an hour with a through draught, to keep the room’s air turning over across a three-hour session, and to carry away the ammonia smell of a rapid fixer.

The darkroom’s standing contradiction applies — light-tight and ventilated at once — and the answer is a light-trapped vent or a baffled extractor rather than an open door. Part II’s laboratory layout lesson owns it.

Materials

Item	Quantity	Note
Film, one emulsion batch	one 36-exposure 135 cassette, or four 135 mm strips from a bulk tin	The batch number goes in the notebook. Use the same emulsion the rest of the part will use
Transmission step wedge	1	Stouffer T2115 or equivalent: 21 steps at a nominal 0.15 to a maximum density of 3.05, on a piece 12.7 × 127 mm
Opaque card	a strip 15 × 40 mm	Masks one end of every strip so it carries its own base-plus-fog patch
Plain glass, 3 mm, edges taped	one piece about 150 × 100 mm	Holds wedge and film in contact. Clean both faces before every exposure
Negative filing sheets or glassine envelopes	4	Two 135 mm strips fit head to head in one 35 mm sleeve pocket
Card label slips	4	Written in pencil or a pigment pen

Chemicals

Nothing is weighed. Every solution is diluted from a stock or a concentrate, following the SOP for mixing from a stock.

Chemical	Quantity	Form
D-76 or ID-11 stock, from Part VIII’s lab — metol, hydroquinone, sodium sulfite, borax	300 ml	Solution, diluted 1+1 immediately before use and discarded after one batch, as Kodak’s J-78 sheet directs
Plain water rinse at 20 °C, or a citric acid stop at 1+19	600 ml, in two changes, or 300 ml	ILFORD note that a water rinse may be substituted for ILFOSTOP but increases the risk of processing marks and stains
Rapid fixer, ammonium thiosulfate type, or a sodium thiosulfate fixer	300 ml at 1+4	Fixed for twice the clearing time, measured on a scrap of the same film
Wetting agent	300 ml at 1+200	Final rinse. ILFORD warn that both too little and too much give uneven drying

The silver bromide that the whole exercise is about is in the emulsion and never in your hands.

Equipment

The light source. Your enlarger, with a negative carrier that is empty and clean, the lens set to a middling aperture, the head at a measured height, and the lamp allowed to warm for five minutes before the first exposure and left on between exposures. A timer that repeats to 0.1 s.

The contact sandwich. The taped glass, the wedge, the film, and a flat dark baseboard. A piece of black card under the film kills the reflection off a white baseboard, which would otherwise add a diffuse second exposure through the back of the film.

A lux meter reading to 1 lx at the levels your enlarger produces, or a phone application whose sensor you have at least characterised, or a spot meter reading in 1/3 EV. Any of the three; all three is better, because the point of this session is partly to find out what each one can resolve.

The wet bench. A daylight developing tank of the size you own; measuring cylinders; a water bath; a thermometer reading to 0.1 °C, checked by the balance and thermometer SOP; tongs; a drying line; a lightbox with an even, colour-stable source.

Estimated cost

££, almost all of it the wedge, and the wedge is an instrument rather than a consumable: bought once, used for the rest of the course and for as long afterwards as you keep it clean. The film is a few frames. The planner carries the shopping list.

!The course has not verified a United Kingdom price for a 21-step wedge

Stouffer’s own product page specifies the parts — the T2115 at 21 steps of 0.15 to a maximum density of 3.05 — but this course has not confirmed a current price or a United Kingdom supplier, so no figure appears here or in the planner for it. Budget for it as a one-off instrument.

Estimated consumables cost

Almost all of the band above is the wedge, and the wedge is an instrument: bought once and used for the rest of the course. Per run, this session consumes four strips of film and four solutions diluted from stock.

Consumed	This session	Sourced price	Cost this session
Film, one emulsion batch	one 36-exposure cassette, or four 135 mm strips from a bulk tin	£6.37–£11.40 per one 35 mm roll, 36 exposures	£6.37–£11.40
D-76 or ID-11 stock, from Part VIII	300 mL, diluted 1+1 and discarded after one batch	Costed in Part VIII’s mixing lab	—
Rapid fixer concentrate	60 mL, to make 300 mL at 1+4	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£1.26–£1.56
Stop bath concentrate, or a plain water rinse	15 mL for 300 mL at 1+19, or two water changes	£10.66–£12.18 per 500 ml of citric acid concentrate, diluted 1+19	£0.32–£0.37
Wetting agent	1.5 mL, for 300 mL at 1+200	£28.70 per 1 L of concentrate, diluted 1+200	£0.04
Negative filing sheets and card label slips	4 pockets, 4 slips	None. A named price gap: sleeving that passes the Photographic Activity Test	—
Opaque card and taped cover glass	one card strip; the glass is kept	None. card-and-paper-stock carries a cost band and no dated figure	—

The priced rows come to £8.00 to £13.37 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 2 of the 7 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

The 21-step wedge is deliberately absent from the table. The course has not confirmed a current price or a United Kingdom supplier for one, the planner names it as a gap, and it is a one-off instrument rather than a per-session cost — so it belongs to the band above and to the equipment list, not here.

Waste streams

Three, kept apart, each labelled per the container SOP and routed per the general waste SOP.

1. Spent developer, alkaline, carrying nearly all of the metol and hydroquinone that were weighed out in Part VIII, because a single strip reduces almost no silver. Its own container.
2. Spent fixer, and the first wash after it, which are silver-bearing and go to the silver stream. This is the stream that matters environmentally and it is the one people tip away.
3. Rinse and wash water. The rinse following the developer carries developer with it and joins stream 1; the first change after fixing joins stream 2; the rest goes with the general stream.

Alternative route

The session as written needs total darkness, because a panchromatic film has no safelight. FOMA say it in one line for FOMAPAN 100: infrared light, or total darkness.

The orthochromatic route. Run the session on ILFORD ORTHO Plus. ILFORD state that its blue and green sensitivity allows handling under a deep red safelight, and recommend the ILFORD 906 filter with a 15 W bulb at not less than 1.2 m. Loading the wedge sandwich, placing the mask and getting the film the right way up then happen in dim red light, which removes most of the fumbling and a good deal of the error from the awkward part of this page.

Two things it costs. The film’s speed differs by illuminant — ILFORD rate ORTHO Plus at ISO 80/20 to daylight and ISO 40/17 to tungsten, which is a full stop, and your enlarger is a tungsten source — so take the tungsten figure when you set the pilot bracket. And the development times are ORTHO Plus’s own, from its own sheet, so the D-76 figures below do not apply and must be replaced before you start.

Whichever film you choose, choose it for the whole part, because the archived strip is meant to be compared with the next one.

⚠A red safelight is not a licence to leave the film out

ILFORD define a safelight as illumination that does not cause a significant visible change to the material during use, and note in the same sentence that the word “safe” is relative. Kodak put the underlying physics plainly in K-4: the colour sensitivity of most emulsions does not end abruptly at a wavelength, most materials retain some sensitivity to the colours a recommended filter transmits, and safelight exposure should therefore always be minimised.

For a sensitometric strip this matters more than it does for a print, because a fog of 0.02 that nobody would see on a picture is a measurable shift in base plus fog and lands directly in your speed point. Run the fog test in the procedure below before you trust any figure from this session.

Preparation

A week before, if the developer stock is not already mixed, mix it: it needs time to settle and this session is not the place to discover a mixing error.

The day before.

1. Clean the wedge on both faces with a lens cloth. Inspect it against the lightbox for scratches and for the fingerprints of your predecessors, and note any on the record — a damaged step is a permanently wrong reading and it is better to know which one.
2. Cut and tape the glass. Cut the mask card.
3. Rehearse the whole handling sequence with the room lights on and the film absent: open the cassette, cut a strip, notch it, lay it on the baseboard, place the wedge, place the mask, place the glass, make the exposure, put the strip in the tin. Do it until it is boring. That rehearsal is the single highest-value twenty minutes on this page.
4. Set out the wet bench and mark the position of every vessel with tape so that it can be found in the dark.

On the day, with the lights on.

5. Set the enlarger head to a height that projects an even circle comfortably larger than the wedge — a 4×5 or 6×6 negative-carrier setting is convenient even for 35 mm film, because the empty carrier throws a wider cone. Measure the height from the baseboard to the lens board and write it down.
6. Stop the lens down to about two stops from wide open, which is where most enlarging lenses are evenest across the field, and write the aperture down.
7. Warm the lamp for five minutes and leave it on for the whole session. A cold tungsten lamp changes both its output and its colour in the first minutes, and every exposure you make while it is drifting belongs to a different light source.

The contact sandwich under the enlarger, and where the meter goes

1. 1Lamp house, empty carrier, lens — lit five minutes before the first exposure and left on; aperture about two stops from wide open
2. 2Measured head height — written down, because a head re-set to a mark is not at the same height
3. 3Black card on the baseboard — kills the reflection back up through the film; a white baseboard gives a second diffuse exposure
4. 4Film, emulsion up — the emulsion is the dull side; find it by touch at a corner in the dark
5. 5Wedge, emulsion down — emulsion to emulsion. Turned over, its base separates the two and the step edges soften
6. 6Glass, 3 mm, edges taped — the weight is what makes the contact; a warped sheet lifts the middle
7. 7Mask card at the tail — taped to the glass, not laid on the film, so it cannot move during the exposure
8. 8Three meter positions — centre and both ends of the wedge line; the falloff between them is your first uncertainty

Layer thicknesses are drawn readable rather than to scale: the film base is about 0.13 mm, its emulsion a few thousandths, and the wedge's polyester base about 0.18 mm, against 3 mm of glass. The one thing drawn honestly is the order.

Measuring the uniformity, which is the step everybody skips

The wedge is 127 mm long. If the illumination falls off across that length, the falloff adds itself to the wedge’s densities and you will read it as film behaviour.

8. Lay the lux meter’s sensor at the centre of where the wedge will sit. Read the illuminance.
9. Move it to each end of that line and read again. Record all three.
10. Convert the worst deviation to wedge steps:

n = log₁₀(E_centre ÷ E_edge) ÷ 0.15

Falloff expressed in wedge steps

E is illuminance and n is the number of nominal wedge steps that the falloff is worth. A reading of 1,000 lx at the centre and 940 lx at the end is log₁₀(1.064) = 0.027, which is 0.18 of a step — negligible. A reading of 1,000 and 800 is 0.097, which is 0.65 of a step, or a third of a stop across the strip, and that is not negligible at all: raise the head, open up the field, or re-orient the wedge across the short axis of the illuminated circle rather than the long one.

Write the falloff down as a number. It is one of the two uncertainties this session actually knows about, and a result quoted without it is a result quoting a precision it does not have.

Procedure

Part 1 — the pilot bracket

The base exposure has to put the whole ten-stop wedge somewhere useful: the clear end of the wedge into the shoulder, the dense end into the toe. Guessing costs a strip; bracketing costs three and tells you where you are.

1. In total darkness, cut and notch four strips of 135 mm as Part IX’s lab sets out, and put three in the light-tight tin.
2. Build the sandwich on the fourth: film emulsion side up, wedge emulsion side down onto it, mask card over the last 15 mm of the film at the tail, glass on top. Emulsion to emulsion is the correct orientation and it matters: with the wedge’s base between the two emulsions, light spreads in the 0.18 mm of polyester before it reaches the film and the step edges soften. It will not change a step’s mean density much; it will make the boundary between steps harder to find when you read them.
3. Expose for 4 s. Return the strip to the tin.
4. Repeat on two more strips at 8 s and 16 s.

!Keep every exposure inside the reciprocity-valid region

ILFORD state that exposure times of one second or less need no reciprocity compensation, and give corrected time as metered time raised to a film-specific power for anything longer — 1.26 for FP4 Plus, 1.31 for HP5 Plus. A 4 to 16 s exposure is therefore already in the region where the correction applies, and it applies unequally along the wedge, because the dense end of the wedge is delivering a far lower illuminance than the clear end for the same duration.

This is a real and awkward limitation of a bright-source contact exposure, and it is the reason Part XIV builds a sensitometer with a shutter fast enough to sidestep it. For this session, do two things and state both. Get the exposure as short as your enlarger and timer allow — open the lens up if you must, and accept slightly worse uniformity for a much shorter time. And write on the record that the exposures were of order several seconds, so that a curve compared with a manufacturer’s — made in a sensitometer at a fraction of a second — is compared knowing that the two exposures were not the same kind of event.

Part 2 — the safelight and light-leak check

5. On the fourth strip, with the room in its working state, give no enlarger exposure at all. Leave it on the baseboard, masked at the tail as usual, for 4 minutes, then cover half its length with card and leave it for another 4. Process it with the others.
6. If the uncovered half is denser than the covered half, you have a light leak or a safelight problem and every other reading in the session is suspect. This ordering follows ILFORD’s own safelight test procedure, which gives the safelight exposure after the enlarger exposure precisely so that the test measures the material in the state it is actually handled in.

Part 3 — processing

7. Bring every solution to 20.0 °C in the water bath and hold it there. Record the temperature at the start, the middle and the end.
8. Load the three exposed strips and one unexposed control strip from the same batch into the tank together. The control strip is not optional and it is not a spare: base plus fog is the floor every density on this page is measured from, the speed criterion is defined as a density above it, and a figure borrowed from a datasheet or from last month’s strip imports somebody else’s developer into your reading.
9. Develop in D-76 or ID-11 1+1 at 20 °C for the time the film’s own datasheet gives — for FP4 Plus at EI 125 in D-76 1+1, ILFORD publish 11 minutes in a spiral tank at 20 °C with intermittent agitation. Use one agitation script and write it in words: “continuous for the first 30 s, then four inversions in 10 s at the start of each minute” is a script; “agitate occasionally” is not.
10. Rinse, fix for twice the clearing time measured on a scrap of the same film, wash, wetting agent, hang to dry without squeegeeing. All four of those steps are Part VI’s and are not repeated here.

Part 4 — reading the steps

Three methods, in increasing order of what they cost and buy. Do at least two, so that you have something to compare.

Lux-meter densitometry. Put the enlarger back on, at any convenient height, and stop down until the meter reads comfortably mid-scale with nothing in the beam. That reading is E₀. Lay the strip on the sensor, step by step, and read E.

D = log₁₀(E₀ ÷ E)

Transmission density from two illuminance readings

Its limit is the meter’s own resolution at the bottom end. A meter reading to 1 lx with E₀ = 1,000 lx can distinguish 10 lx from 11 lx, which is 0.04 of density at D = 2.0, but at D = 2.7 it is reading 2 lx against 3 lx and the density step is 0.18. Find your own floor: read a step near the dense end five times, moving the strip between readings, and take the spread. That spread, in density, is the resolution of your method, and no difference smaller than it may be reported as real.

Spot-meter densitometry. Put the strip on the lightbox and meter each step. One EV is a factor of two in luminance, which is 0.301 in density.

D = 0.301 × (EV₀ − EV_step)

Density from a spot meter

A meter resolving 1/3 EV therefore resolves 0.10 of density and no better, which is two-thirds of a nominal wedge step — coarse, but instant, and honest if you say so. Its other limit is the measuring angle: a 1° spot at the working distance may be wider than a step.

Visual matching. Lay the strip and the wedge side by side, touching, on the lightbox, and find the wedge step whose grey matches each strip step. The wedge step’s nominal density is then your reading. This is the method Part IX described and its three limits stand: the wedge is nominal rather than calibrated, you are matching developed silver against a manufactured neutral, and the eye compares well only when the two patches touch and are lit alike.

Part 5 — converting a step number into a log exposure

This is the arithmetic that turns a strip into a curve, and it is one line.

log H(step k) = log H₀ − D_wedge(k)

Log exposure at a wedge step

log H₀ is the log exposure the enlarger delivered to bare film in the time you gave, and D_wedge(k) is the nominal density of wedge step k. Kodak’s workbook does exactly this subtraction in its own worked example.

For a T2115, step 1 is 0.05 and every step adds a nominal 0.15, so D_wedge(k) = 0.05 + 0.15(k − 1) and the twenty steps span 3.00 log units — ten stops on one piece of film.

You do not know log H₀ in absolute lux-seconds unless you measured the illuminance at the film plane and multiplied by the time, and you should do exactly that if your meter allows it. If it does not, set log H₀ = 0 and plot against relative log exposure, which loses nothing except the ability to compute a speed number, and say on the plot which you did.

Expected observations

A wet strip that looks wrong. Wet film is milky and the shadow steps look far denser than they will be dry. Judge nothing until it is dry.

A clear step, a black step, and about twelve you can tell apart. With a well-chosen exposure the clear end of the strip has two or three steps that all look equally black — the shoulder — and the dense end has two or three that all look equally clear against the masked patch — below the toe. The useful run in between is where the film is actually recording, and on a normally developed 35 mm film it is commonly twelve to sixteen steps.

The masked patch not quite matching clear film. It should be slightly denser than a piece of the same film simply fixed without development. That difference is the development fog, and it is small: a tenth or less on a fresh film in a fresh developer.

Steps whose boundaries are soft rather than sharp, if the wedge was the wrong way up. Compare an emulsion-to-emulsion strip with a base-to-emulsion one once, deliberately, and you will never wonder about it again.

The three bracket strips looking like the same curve slid sideways, which is exactly what they are: a bracket in time moves the whole strip along the exposure axis without changing its shape, provided reciprocity is holding. If the three do not look like translations of each other, reciprocity is not holding, and that is worth knowing.

What is happening chemically

Nothing on this page is new chemistry; what is new is that you are about to put numbers on it.

Exposure sorts the grain population. Photons absorbed by a silver bromide crystal free electrons that reduce interstitial silver ions to silver atoms, and a few atoms clustered at one site make a latent-image centre big enough to catalyse development. Whether a given grain gets one depends on its size, its sensitisation and how many photons happen to land on it, so at the dense end of the wedge only a small minority of grains carry a developable centre. That minority, as a function of exposure, is the toe.

Development amplifies, enormously and selectively. The developing agents reduce the whole crystal to metallic silver once a centre gives them a foothold, an amplification Part IV puts at a factor of the order of 10⁸ or 10⁹. A grain either develops or does not; the density is the count of developed grains times the covering power of each. Density is therefore not proportional to the exposure but to the fraction of the population recruited, which is why the curve has a shape at all.

The straight line is proportional recruitment. Over the middle of the exposure range, each 0.30 of log exposure recruits a roughly constant increment of the population, so density rises linearly with log exposure. The slope of that line is what development time controls: longer development completes more of each grain, and a few more marginal grains, so the same recruitment yields more silver.

The shoulder is running out of grains. Once nearly every developable grain has developed, more exposure has nothing left to recruit, and density flattens towards D_max.

Fog is development without exposure. The control strip’s density above clear film is grains reduced by the developer with no latent-image centre at all, plus the film base’s own density and any dye in it. It rises with development time, with developer activity and with the film’s age, and it is the floor everything is measured against, which is why it is measured every time and never assumed.

Data to record

Copy this into the notebook before the session, not after it — or print the course’s characteristic-curve record, whose three sheets are this section in the order the work happens. SN-1 goes to the enlarger and is filled before the film is developed, because the head height, the aperture, the three illuminance readings and the wedge’s orientation cannot be reconstructed afterwards. SN-2 is the twenty-one readings with the masked patch and the control strip as rows of their own. SN-3 is what the plotted curve says, and its last field asks for the whole finding in one sentence.

Session header. Date; who; room temperature at start and end; the lightbox and the meter used, with its checked offset; the wash water temperature.

Light-source block. Enlarger make and head type, condenser or diffusion; lamp type and power; time the lamp had been on before the first exposure; measured head height; lens and aperture; the three uniformity readings and the falloff in wedge steps; the exposure times of all three bracket strips.

Wedge block. Part number; whether it is a calibrated part; the density of step 1 and the nominal increment; any damage noted; the orientation used, emulsion to emulsion or not.

Film and process block. Film, format, emulsion batch; the formula version of the developer stock and its mixing date; dilution; volume; time; temperature at start, middle and end; the agitation script in words; fixer, dilution, measured clearing time and fixing time; wash; drying.

Readings. One table per strip: step number, nominal wedge density, log exposure, and a density column for each reading method you used. Plus the masked patch and the control strip, in every column.

Uncertainties, as measured today. The illumination falloff in steps; the repeat spread of your reading method at a dense step and at a thin one; the meter’s own resolution at the levels used.

Deviations. Everything that went differently from the plan.

Analysis

1. Plot on equal scales. 0.30 of density must occupy the same distance as 0.30 of log exposure, or the contrast-index construction does not work. Two centimetres per 0.30 fits A4.
2. Draw base plus fog first, from the control strip, as a horizontal line across the whole plot.
3. Plot all twenty-one points, then draw a smooth curve through the scatter rather than through every point. Mark the steps you could not separate at either end and exclude them from the curve — they are the limits of your reading, not the shape of the film.
4. Locate the criterion point m at D₀ + 0.10, and read log H at it.
5. Test the development condition: the density 1.30 log units to the right of m must exceed the density at m by 0.80 ± 0.05. If it does, you may quote a speed under the course criterion; if it does not, you have a relative speed and a development that is not at the criterion, and both facts go in the record.
6. Compute the contrast index by the arithmetic form of the course convention, and the gamma of the straight run, and the average gradient over the whole readable strip. Report all three with their construction named.
7. Compare with the manufacturer’s published curve for the same film and developer, and compare it properly: theirs is at a stated development in a sensitometer under a specified illuminant, read on a densitometer of a specified geometry; yours is at your development under a tungsten enlarger read with a lux meter. Compare shapes and slopes, not positions. If your contrast index is within 0.05 of the value their development time is meant to produce, your whole chain is working.
8. Attach the uncertainties to the numbers. “Contrast index 0.59, measured under the criterion defined in Part XIII, from a strip read by lux-meter densitometry with a measured repeat spread of 0.03 in density and an illumination falloff of 0.2 wedge steps across the strip.” That sentence is the deliverable of this page.

📓Everything computed from a nominal wedge is relative

Part IX established it and this page inherits it: Stouffer state that the calibrated parts are the T2120CC and T1530CC, measured against NIST Standard Reference Material 38120C on a densitometer conforming to ANSI PH2.19-1986. An ordinary T2115 is manufactured to the increment, not certified to it.

So every log exposure on your plot is a nominal 0.15 multiplied by a step count, every contrast index is a slope measured against that nominal axis, and every speed is relative. What survives is comparison: this developer against that one, this month against last, your film against your own film — all read against the same wedge, whose errors are then common to both sides and cancel. That is not a consolation prize. It is the whole of what a contrast target needs to be good for.

What a strip from this session looks like plotted the wrong way round, and why you plot it the other way

• Film density against wedge density, one strip

Show the numbers behind this plot

Twenty-one points plotted against the wedge's own density scale, running from 0.00 at the left to 3.00 at the right. Film density falls from 1.62 at the left, where the wedge is clearest and the film received most light, to 0.13 at the right, where the wedge is densest and the film received least. The shape is a characteristic curve reflected left to right: the shoulder is now at the far left and the toe at the far right, and the last five or six points at the right lie flat at base plus fog. The lesson of the diagram is that the wedge scale runs backwards to the exposure scale, so plotting step number or wedge density along the bottom gives a mirror image of the curve, and the fix is to subtract the wedge density from the log exposure delivered to bare film.

Series	Nominal wedge density at the step (increasing = less light reaching the film)	Density on the film
Film density against wedge density, one strip	0.00	1.62
Film density against wedge density, one strip	0.15	1.55
Film density against wedge density, one strip	0.30	1.46
Film density against wedge density, one strip	0.45	1.35
Film density against wedge density, one strip	0.60	1.23
Film density against wedge density, one strip	0.75	1.10
Film density against wedge density, one strip	0.90	0.96
Film density against wedge density, one strip	1.05	0.82
Film density against wedge density, one strip	1.20	0.68
Film density against wedge density, one strip	1.35	0.55
Film density against wedge density, one strip	1.50	0.43
Film density against wedge density, one strip	1.65	0.33
Film density against wedge density, one strip	1.80	0.25
Film density against wedge density, one strip	1.95	0.20
Film density against wedge density, one strip	2.10	0.17
Film density against wedge density, one strip	2.25	0.15
Film density against wedge density, one strip	2.40	0.14
Film density against wedge density, one strip	2.55	0.14
Film density against wedge density, one strip	2.70	0.13
Film density against wedge density, one strip	2.85	0.13
Film density against wedge density, one strip	3.00	0.13

An illustrative shape, not a measurement. Wedge density and log exposure run in opposite directions, so this plot is a characteristic curve mirrored. Subtracting the wedge density from log H₀ turns it the right way round, and the same twenty-one points then look like every curve in this part. 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.

Troubleshooting

What you see	Likely cause	What to do
Every step is black, including the densest	Far too much exposure, or the wedge was not in the beam	Check the wedge was actually under the glass, then divide the time by four and re-bracket. Two steps is one stop, so a strip that is four steps too dark needs half the time
Every step is clear except two or three at the wedge’s clear end	Far too little exposure	Multiply the time by four, or open the lens two stops, and re-bracket. Prefer opening up: it keeps the exposure short and reciprocity nearer to holding
The masked patch is as dense as the first exposed step	A light leak, or safelight fog, or the mask moved	Read the no-exposure strip from part 2. If its two halves differ, the safelight is the cause; if they match each other but are both dense, it is a leak or a fogged film
Density falls off along the strip in the same direction on every strip	Illumination falloff, not film behaviour	Go back to the uniformity measurement. If the falloff exceeds about a third of a step, raise the head or re-orient the wedge and re-expose
Step boundaries are soft and hard to place	Wedge base against film emulsion instead of emulsion to emulsion, or poor contact under the glass	Turn the wedge over. Weight the glass evenly; a warped sheet lifts the middle
The curve has a kink in the middle that the bracket strips do not share	A mis-read step, or a scratch or fingerprint on one wedge step	Re-read that step. Then hold the wedge to the lightbox and look at it
Readings drift steadily while you work through the strip	The enlarger lamp warming or the lightbox drifting, or your eyes adapting	Re-read step 1 at the end of the run. If it has moved, the source has drifted; interleave a reference step every five readings from then on
The three bracket strips are not translations of one another	Reciprocity failure across the bracket, or the lamp changed	Recompute the expected shift: two steps per stop. If the strips fall short of that, low-intensity reciprocity failure is the likely cause and the exposures were too long
Milky or violet cast on the dried strip	Under-fixing, or an exhausted bath	Re-fix in fresh fixer. Do not archive an under-fixed strip; Part XV will read it in months and by then the damage is its own

Clean-up

Pour each solution into its own labelled waste container as you finish with it, rinse the vessel twice into the same container, and only then rinse it into the sink. Wash and dry the tank, the reels, the cylinders and the tongs, keeping developer and fixer utensils apart — Kodak name mixing equipment that has not been thoroughly cleaned as a cause of solution contamination, and a developer with fixer in it is a session lost. Clean the wedge and the glass and put the wedge away in its sleeve. Throw the gloves away. Close the laboratory to the SOP.

Storage

The wedge goes back into its own sleeve, flat, out of the light, and is handled by its edges for the rest of its life. It is the reference every strip in this part is measured against, and a scratch on it is a systematic error in everything measured afterwards.

The strips go into a sleeve with a card slip carrying the eight lines Part IX’s lab specifies, with this session’s light-source block added. The test the label has to pass is unchanged: could somebody holding only this sleeve repeat the strip?

The data sheet goes with them, in the same pocket, because a strip whose exposure geometry has been lost is a piece of grey film.

Unused film goes back in its packaging, cool and dark, batch number legible.

Disposal considerations

Three chemistries leave this bench and none of them belongs in a drain.

Spent developer is alkaline and carries almost all of the metol and hydroquinone it was mixed with, because a single strip reduces a negligible mass of silver. Both agents are notified as very toxic to aquatic life with long-lasting effects, which is the property that governs how the stream is handled.

Spent fixer and the first wash after it carry silver as thiosulfate complexes. Silver in that form is the environmentally significant part of photographic waste and the reason silver recovery exists as an industry.

Rinse and wash water carries whatever the step before it carried, in less concentration and no different in kind.

The chemistry says keep them apart and keep them contained; the route is the course’s disposal page, and the procedure — which container, in what order, labelled how — is the silver-bearing waste SOP and the general chemical waste SOP, which this page cites rather than restates. ILFORD’s own guidance for United Kingdom domestic users is to take used chemistry to a household waste and recycling centre. Local regulation governs, and it varies: check your local regulations before you dispose of anything from this session.

Questions

1. Your wedge is a T2115 and you exposed for 8 s at f/8. What is the log exposure difference between step 3 and step 17, and how many stops is that?
2. Your control strip reads 0.14 and step 12 of your exposed strip reads 0.24. Is step 12 the criterion point? What else do you need before you can say so?
3. The illuminance at the centre of your field is 820 lx and at one end of the wedge it is 690 lx. How many wedge steps is that worth, and does it matter?
4. You read the same dense step five times and get 2.41, 2.55, 2.38, 2.60, 2.47. What is your resolution at that density, and what is the smallest contrast-index difference you may report from this strip?
5. Your curve gives a contrast index of 0.71 where the datasheet’s development time was meant to produce 0.62. Name three explanations that do not involve the film being different from its specification, and say how you would distinguish them.
6. Why does this page insist on an unexposed control strip in the same tank rather than a base-plus-fog figure from the datasheet?

Further experiments

Three development times, one afternoon. The single highest-value extension of this page. Expose nine strips from one geometry in one sitting, develop three at each of three times, and plot contrast index against time. You then own the graph the whole part was built to let you draw, for your film in your developer, and you can read a development time for any contrast you want for as long as that film exists.

The same wedge, both enlarger heads. If you have access to a condenser and a diffusion head, expose and process identical strips under each and read them. The difference you find is not in the film; it is the Callier effect in your reading, because a lux meter under a condenser sees a more directional beam than one under a diffuser. It is the cheapest demonstration in the course of why densitometry has a specified geometry.

Bracket in aperture instead of time. Two strips, one at f/5.6 for 4 s and one at f/8 for 8 s. They receive the same nominal exposure by two different routes. If they do not give the same curve, reciprocity is the first suspect, and lens vignetting the second.

Read the same strip three ways and plot the three against one another. Lux meter, spot meter and visual matching on the same twenty-one steps. Where the three agree you have a density; where they diverge you have found the limit of the cheapest of them, and that boundary is worth more to you than any single reading.

Keep the strip for Part XV. When the densitometer exists, read this strip again and plot its densities against today’s. The slope of that plot is your interim method’s calibration, and its scatter is its precision — measured rather than estimated, a year after you needed it.

Check your understanding

Question 1. You expose a 21-step wedge onto film for 8 s, and the resulting strip shows separation from step 4 to step 16. You want the separation to start at step 2 instead. What exposure do you give, and what assumption does the answer depend on?

• Halve the time to 4 s; it assumes nothing
• Multiply the time by 10 to the power 0.30, which is 2.0, giving 16 s; it assumes the reciprocity law is holding over the change
• Double the aperture number to f/16; it assumes the lens is linear
• The exposure cannot be changed without changing the wedge

Show the answer and why

Answer: Multiply the time by 10 to the power 0.30, which is 2.0, giving 16 s; it assumes the reciprocity law is holding over the change

Moving the threshold two steps down the wedge means giving two steps more exposure, and a nominal step is 0.15 in log exposure, so two steps is 0.30 — exactly one stop, a factor of 10 to the power 0.30, which is 2.0. Doubling the time from 8 to 16 s does it. The assumption is the awkward part: the correction is exact only if the reciprocity law holds across the change, and ILFORD state that compensation is unnecessary only at one second or less. At these durations the film is already in low-intensity reciprocity failure, unequally along the wedge, so the real shift will be slightly less than two steps. That is why opening the lens is often the better move: it keeps the time short and the arithmetic closer to true.

Question 2. Your control strip reads 0.16 and your curve reaches 0.26 at a step whose log exposure is 0.75. The density 1.30 log units further along is 0.98. What may you report?

• A speed of 800 divided by 10 to the power 0.75, quoted as an ISO speed
• A relative speed only, because the rise of 0.72 over 1.30 falls below the 0.75 to 0.85 window the criterion allows
• Nothing, because the base plus fog is too high
• A contrast index of 0.72

Show the answer and why

Answer: A relative speed only, because the rise of 0.72 over 1.30 falls below the 0.75 to 0.85 window the criterion allows

The criterion point is right: 0.16 plus 0.10 is 0.26, at log H 0.75. But the criterion has a second condition, on the development, and this fails it — 0.98 minus 0.26 is 0.72, where the window is 0.80 with a tolerance of 0.05. The strip is under-developed for the purpose. The exposure at the criterion point is still a perfectly good relative speed for comparing with another strip read the same way, and the useful next step is to read the development time that would satisfy the condition off a time plot. As for the first option, no page of this course may attribute a figure to a standard it does not quote, and this figure came off a nominal wedge under a tungsten enlarger read with a lux meter, so it is not an absolute speed by any route.

Question 3. Why does the procedure ask you to measure illuminance at the centre and at both ends of where the wedge will sit, before any film is exposed?

• To set the exposure time correctly
• Because illumination falloff across the wedge adds itself to the wedge densities and would be read as film behaviour, and expressing it in wedge steps says immediately whether it matters
• To check the lamp is working
• Because the lux meter needs calibrating against the enlarger

Show the answer and why

Answer: Because illumination falloff across the wedge adds itself to the wedge densities and would be read as film behaviour, and expressing it in wedge steps says immediately whether it matters

A step wedge works because every step differs from its neighbour only in how much light it passes. If the source itself is brighter at one end of the strip than the other, that gradient is indistinguishable from a wedge gradient and lands in your curve as a change of slope. Converting it to steps makes the decision easy: divide the logarithm of the centre-to-edge illuminance ratio by 0.15, and if the answer is much above a third of a step you have a systematic error worth a third of a stop across the strip. The fix is geometric — raise the head, use a wider carrier, or lay the wedge across the short axis of the illuminated field — and it costs nothing if you find it before you expose rather than after.

Question 4. A lux meter reading to the nearest 1 lx is used with E-zero set to 1000 lx. Roughly what density difference can it resolve at a film density of 2.7, and what follows?

• About 0.01, so it resolves everything on the strip
• About 0.18, because 2.7 of density leaves only about 2 lx to measure and the next resolvable reading is 3 lx
• About 0.30, because one stop is 0.30
• It cannot read above a density of 1.0 at all

Show the answer and why

Answer: About 0.18, because 2.7 of density leaves only about 2 lx to measure and the next resolvable reading is 3 lx

A density of 2.7 transmits 10 to the power minus 2.7, which is 0.002, so 1000 lx becomes about 2 lx. A meter quantising to 1 lx can then only tell 2 from 3, and the logarithm of 3 over 2 is 0.18 of density. That is more than a whole nominal wedge step, so the dense end of the strip is effectively unreadable by this method and those points must be excluded from the curve rather than plotted as though they were data. Two remedies exist and both are worth knowing: raise E-zero so there is more light to divide, which the enlarger lets you do by opening the lens, or accept the limit, say so, and read the dense end again in Part XV when a densitometer exists.

Question 5. What is the strongest argument for exposing the whole bracket in one sitting from one unmoved geometry, rather than one strip per evening?

• It saves film
• Everything that would otherwise vary between sittings — lamp output, head height, aperture, mains voltage, the wedge position — is held identical, so a difference between strips can only come from the variable you changed
• The developer keeps better
• The wedge changes density over time

Show the answer and why

Answer: Everything that would otherwise vary between sittings — lamp output, head height, aperture, mains voltage, the wedge position — is held identical, so a difference between strips can only come from the variable you changed

This is the same discipline Part IX built its whole exposure batch around, and it is the difference between an experiment and a collection of observations. A tungsten lamp drifts as it ages and with mains voltage; a head re-set to the same marked height is not at the same height; a wedge laid down again is not in the same part of the field. Each of those is small, and together they are comfortably a wedge step, which is half a stop — larger than most of the differences you are trying to measure. Exposing everything at once converts all of them from variables into constants, and the price is only that the strips then wait, undeveloped, with their latent images slowly ageing, which is a smaller and better-understood error.

Question 6. Your measured contrast index is 0.59 and the manufacturer says the development time you used should give 0.62. Which conclusion is best supported?

• The film is out of specification and should be returned
• The chain agrees to within its own uncertainty, since the wedge is nominal, the reading method has a measured spread of a few hundredths, and the two figures were not measured the same way
• The developer is exhausted
• The contrast index convention used by the course is wrong

Show the answer and why

Answer: The chain agrees to within its own uncertainty, since the wedge is nominal, the reading method has a measured spread of a few hundredths, and the two figures were not measured the same way

Three separate sources of difference sit between the two numbers before any film behaviour is reached. The wedge is manufactured to its increment rather than certified to it, so the exposure axis is nominal. The reading method has a repeat spread you measured today, typically a few hundredths of density, which propagates into the slope. And the manufacturer figure comes from a sensitometer, a specified illuminant and a densitometer of specified geometry, none of which you have. Against all that, 0.59 against 0.62 is agreement, and reporting it as such — with the uncertainties attached — is the honest reading. A difference worth investigating would be one several times your measured spread, and the way to find out is to repeat the strip, not to reason about it.

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