Paper Sensitometry: Reflection Density, Grades and ISO(R)

A negative’s curve climbs for three log units and stops when it runs out of silver. A paper’s curve climbs for one log unit and stops when it runs out of black, and the black it runs out of is the darkest thing a sheet of coated paper can do while still being looked at in a room. Everything odd about paper sensitometry follows from those two sentences.

This lesson is about the second curve in the chain, and about the one number that joins it to the first.

Reflection density: measuring the light that comes back

A negative is measured by the light that goes through it. A print cannot be: it sits on an opaque base, and what you can measure is the light it sends back.

Reflection density is therefore the logarithm of a ratio of reflected quantities rather than transmitted ones — the light returned by a reference white against the light returned by the sample. The arithmetic is identical to transmission density: a reflection density of 1.0 returns a tenth as much light as the reference, 2.0 a hundredth.

Two things about the measurement are not identical, and both matter.

The geometry has to keep the shine out. A glossy print reflects two quite different things: a diffuse return from the paper and the silver in it, which carries the image, and a specular glint off the surface, which carries only the lamp. Measure at the wrong angle and you measure the lamp. The standard answer is to illuminate the print obliquely and look at it straight on, or the reverse, so that the mirror reflection goes somewhere the detector is not. The course’s own convention, stated here, is oblique illumination at about 45° with the detector on the normal, described from the physics rather than quoted from anywhere; where the course later builds a reflection head in Part XV it will state the geometry it actually achieved.

A reflection density needs a reference white, and the reference is not the paper. If the reference were the sheet’s own unexposed base, every paper would read 0.00 at its whitest and the comparison between two papers of different base tone would vanish. The reference is external.

!How this course cites ISO 5, and what it takes from it

The relevant standards are sold rather than published and the course does not purchase them. What it takes from the publisher’s free preview of ISO 5-3:2009 is one structural fact and no content: the ISO 5 series, Photography and graphic technology — Density measurements, has four parts, and transmittance density and reflection density are governed by different parts of it — Part 2 for transmittance geometry, Part 4 for reflection geometry, with Part 3 covering the spectral conditions of both.

That structure is the whole point for a reader here. A transmission density and a reflection density are not the same quantity measured on different materials. They are two quantities, standardised separately, that happen to share a logarithm and a name. Nothing in this course prints a geometry, a tolerance or a value as though quoted from any part of ISO 5, and no figure on this page is described as an ISO density.

Why the axis stops at about 2.2

A film curve is drawn to 3.0 and beyond because film reaches those densities. A paper curve is drawn to about 2.2 because paper does not.

The reason is physical rather than conventional. Maximum black on a print is set by how much light the coated layer can stop from coming back out — and some always comes back, scattered by the paper fibres or the resin coating beneath the emulsion and by the surface itself. ILFORD publish the figure for two generations of one paper on the same 190 gsm resin-coated base: 2.15 for the current MULTIGRADE RC DELUXE and 2.05 for the MULTIGRADE IV RC DELUXE it replaced, and they attribute the difference to the new paper’s greater depth and slightly extended tonal range.

Sit with the size of that number for a moment, because it is the single most consequential fact in printing. A reflection density of 2.15 is a 140 to 1 ratio between paper white and maximum black. Your subject was seven stops, which is 128 to 1, and it was measured in a world that routinely offers ten or twelve. The print is the narrowest part of the whole chain, narrower than the negative by a log unit, and every decision upstream is ultimately a decision about how to fit a wide world into 2.15.

Kodak’s workbook draws its paper curves to exactly this scale, on an axis running to 2.4, and says the three things a reader has to absorb: paper densities are read on a reflection densitometer, papers are much slower than films, and paper speed is not figured the way film speed is.

The shape of a paper curve

Put a film curve and a paper curve side by side and the paper’s is unmistakable: it starts almost flat, rises steeply through a short middle, and stops hard.

Two paper curves, soft and hard, drawn to the same scale

• A soft grade: exposure range about 1.9, so ISO(R) near 190
• A hard grade: exposure range about 0.5, so ISO(R) near 50

Show the numbers behind this plot

Two curves of reflection density against relative log exposure, both rising from a paper white near 0.04 to a maximum black of 2.15. The soft curve begins to lift at about relative log exposure 0.3, rises through a long, moderately steep middle, and reaches its maximum at about 3.0, so the exposure it uses spans about 1.9 log units. The hard curve does not lift until about 0.9, rises far more steeply, and reaches its maximum by about 2.3, so the exposure it uses spans about 0.5 log units. The two curves cross near the middle, at a density around 1.2, which is why changing grade rotates the tonal scale about the mid-tone rather than making the whole print lighter or darker. Both flatten abruptly at the top rather than tapering, because maximum black is a ceiling set by the paper rather than by the exposure.

Series	Relative log exposure	Reflection density
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.00	0.04
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.15	0.05
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.30	0.07
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.45	0.11
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.60	0.18
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.75	0.29
A soft grade: exposure range about 1.9, so ISO(R) near 190	0.90	0.43
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.05	0.60
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.20	0.79
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.35	0.99
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.50	1.20
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.65	1.40
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.80	1.58
A soft grade: exposure range about 1.9, so ISO(R) near 190	1.95	1.74
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.10	1.87
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.25	1.98
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.40	2.05
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.55	2.10
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.70	2.13
A soft grade: exposure range about 1.9, so ISO(R) near 190	2.85	2.14
A soft grade: exposure range about 1.9, so ISO(R) near 190	3.00	2.15
A hard grade: exposure range about 0.5, so ISO(R) near 50	0.70	0.04
A hard grade: exposure range about 0.5, so ISO(R) near 50	0.80	0.05
A hard grade: exposure range about 0.5, so ISO(R) near 50	0.90	0.08
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.00	0.14
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.10	0.26
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.20	0.44
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.30	0.67
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.40	0.93
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.50	1.20
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.60	1.45
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.70	1.67
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.80	1.85
A hard grade: exposure range about 0.5, so ISO(R) near 50	1.90	1.98
A hard grade: exposure range about 0.5, so ISO(R) near 50	2.00	2.07
A hard grade: exposure range about 0.5, so ISO(R) near 50	2.10	2.12
A hard grade: exposure range about 0.5, so ISO(R) near 50	2.20	2.14
A hard grade: exposure range about 0.5, so ISO(R) near 50	2.30	2.15
A hard grade: exposure range about 0.5, so ISO(R) near 50	2.45	2.15

Shapes drawn to teach, not measured from a material; the maximum density of 2.15 is ILFORD's published figure for MULTIGRADE RC DELUXE and the exposure ranges are chosen to bracket their published ISO(R) figures. Note the two ends: a long flat foot where highlights live, and a hard ceiling rather than a taper. 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.

The long toe is where the highlights live. The first part of the exposure scale produces almost no density at all, which is what makes a clean, textured highlight possible — and also why highlight separation is the first thing lost when anything lifts that foot.

The steep middle is short and it is the whole picture. On a hard grade the entire scale from just off white to nearly black occupies half a log unit, which is under two stops. That is not a defect. A paper that took three log units would be so flat that no print made on it could look like anything.

The shoulder is not really a shoulder. A film shoulder tapers because developable grains run out gradually. A paper’s top end stops because it has hit maximum black, which is a ceiling set by the material’s ability to stop light rather than by the supply of silver, and it arrives abruptly. Practically that is why a print’s deepest shadows either separate or do not, with very little in between, and why “there is no detail in the blacks” is a statement about where the exposure put them rather than about the paper’s quality.

Maximum black depends on the surface as much as the silver. A matt sheet scatters light back off its own texture and cannot reach the black a glossy sheet reaches from the same emulsion and the same development. That is a property of the surface, not a failure of processing, and it is why comparing Dmax between two papers of different finish tells you about the finishes.

Log exposure range, and what ISO(R) actually is

Here is the number that joins the two curves of the chain, and it is simpler than its name.

The log exposure range of a paper is the span of log exposure between the point where useful density begins and the point where it stops rising usefully. A paper whose whole scale occupies 1.10 log exposure units has a range of 1.10.

ILFORD’s ISO Range figure is that number multiplied by 100, and they attribute the measurement on their own sheet to ISO 6846:1992. So a published figure of 110 means a log exposure range of 1.10; a figure of 50 means 0.50.

Log exposure range = ISO(R) ÷ 100

What a range figure means

Their instruction for using it is one sentence: take the effective negative density range, multiply by 100, and choose the nearest range figure. Their own worked example takes 1.32 log exposure units to the figure 130.

Here is the published table for MULTIGRADE RC papers, which is worth reading as a picture of what a grade is:

Product	00	0	1	2	3	4	5	None
MULTIGRADE RC DELUXE (current)	160	130	110	90	70	60	50	90
MULTIGRADE IV RC DELUXE	180	160	130	110	90	60	40	110
MULTIGRADE RC WARMTONE	190	160	130	110	90	70	50	110
MULTIGRADE RC COOLTONE	180	160	120	100	80	60	50	100

Read across a row and you are reading the whole meaning of contrast grade: a grade is a length of exposure axis. Filter 00 gives the paper 1.60 log units to work with; filter 5 gives it 0.50. Nothing else about the sheet has changed — same silver, same base, same maximum black — only how much of the world it will accept.

Read down the columns and something else appears. The current MULTIGRADE RC DELUXE at filter 1 has the range MULTIGRADE IV had at filter 2, and ILFORD say as much: in the redesign the low to mid grades were offset up to one grade harder. A grade number is a maker’s designation and not a measurement, and it does not survive a product revision, let alone a change of manufacturer. The range figure does.

📓The one number to carry out of this lesson

If you remember nothing else: negative density range on one side, paper log exposure range on the other, and printing is the business of matching them.

A negative whose density range is longer than the paper’s exposure scale loses one end or the other — blocked highlights, empty shadows, or both. A negative whose range is shorter prints flat and grey and needs a harder grade to stretch it. Choosing a grade is choosing the paper whose exposure scale is the length of your negative’s density range, and everything else in the printing parts is refinement on top of that one match.

Two ways to change the length of that axis

Graded papers: change the box

A graded paper has one emulsion of one contrast, and its exposure scale is what it is. ILFOBROM GALERIE FB is the survivor in ILFORD’s own range: a traditional paper on a double-weight fibre base, supplied glossy in four equally spaced grades 1 to 4 and matt in three. Change the contrast and you change the packet.

The virtue is that each sheet is a single, well-behaved emulsion. The cost is inventory, half-grades you cannot have, and no way to give two parts of one print two different contrasts.

Variable-contrast papers: change the colour of the light

ILFORD’s own account of how MULTIGRADE works is worth following exactly, because the sensitometry falls out of it.

The paper is coated with a mixture of three separate emulsions. Each is a basic blue-sensitive emulsion carrying a different amount of green sensitising dye, so one part responds mainly to blue, one to blue with some green, and one to both. Crucially, all three have the same contrast and the same speed to blue light; only their green speeds differ.

Now expose it two ways.

Blue light. All three parts react, and they react together, because they have the same blue speed. Three identical curves added on top of one another give one curve of the same shape — and the whole image is made over a narrow band of exposure. Narrow exposure range means high contrast.

Green light. Only the most heavily dyed part answers at first; the others need much more light before they contribute. Three curves staggered along the exposure axis, added together, give one long shallow curve. Wide exposure range means low contrast.

⚗Why adding staggered curves lowers contrast

This is worth doing in your head once, because the same argument recurs in emulsion design and in split-grade printing.

Suppose each of the three emulsions alone would give a curve rising from nothing to full density over 0.5 log units. Stack them exactly on top of each other and the sum rises over the same 0.5 — the slope is three times as steep in density but the range is unchanged, and after normalising to the paper’s one available maximum black, the shape is unchanged too. That is the blue case.

Now slide them apart by 0.5 each. The first emulsion does its whole rise, then the second, then the third, and the sum climbs over 1.5 log units instead of 0.5 to reach the same maximum black. Same silver, same emulsions, three times the exposure range and therefore a third of the contrast. That is the green case, and the stagger is nothing but the difference in green sensitising dye.

Everything between the two is a mixture of blue and green light, which is why a variable-contrast paper gives a continuum of grades and not a set of steps.

The filters follow immediately. A magenta filter absorbs green and transmits blue, so magenta is the hard direction. A yellow filter absorbs blue and transmits green, so yellow is the soft direction. Twelve MULTIGRADE filters run 00 to 5 in half steps, and on a colour head the same job is done with dialled yellow and magenta.

Paper speed, and the step at grade 3.5

Papers have speeds, and ILFORD publish them as ISO Speed (P) figures alongside the range figures, again to ISO 6846. Two warnings travel with the table and both are theirs.

A paper speed is not a film speed. ILFORD say so in a note on the sheet, and add the only comparison worth having: MULTIGRADE RC papers correspond to a film ISO of roughly 3 to 6. Against a film at ISO 100 that is between four and five stops slower; against ISO 400, six or seven. Either way it is the reason a print is exposed for seconds under an enlarger while a negative is exposed for a fraction of a second in a camera.

A paper’s speed depends on the filtration. Here is the current table beside the one it replaced:

Product	00	0	1	2	3	4	5	None
MULTIGRADE RC DELUXE (current)	240	240	240	240	240	220	220	500
MULTIGRADE IV RC DELUXE	200	200	200	200	200	100	100	500

The design intent is visible in the numbers. Across filters 00 to 3 the speed is constant, which is what “speed-matched filters” means and why ILFORD can say the exposure time is the same for 00 to 3½: change grade, leave the timer alone, and only the contrast moves. Above 3½ it changes, and on the older paper it changed by a factor of two — a full stop, which is why a generation of printers learned to double the exposure at grade 4 and got a shock every time they forgot.

Why does it change at all? ILFORD publish the effect and not the mechanism, so what follows is the course’s own reading of their own account, offered as an inference rather than as their explanation. The hard grades are made by cutting the green out with magenta filtration, and past about filter 3½ the filter is absorbing a substantial share of the lamp’s total output. The emulsion is no less sensitive; there is simply less light arriving. That reading is consistent with the direction of the change and with the unfiltered column being the fastest of all, and the course has not found a manufacturer statement confirming it.

What ILFORD do state is the outcome, and their redesign attacked exactly this: the current paper is approximately one stop faster than the MULTIGRADE IV it replaced, and no longer has a large step change in exposure between grade 3½ and grade 4, so that only fine tuning is needed when switching between grades. Read the two speed tables against that claim and a wrinkle appears worth noticing: at filters 00 to 3 the published figures are 240 against 200, which is about a quarter of a stop, while at filters 4 and 5 they are 220 against 100, which is a little over a whole one. The “approximately one stop” is most nearly true where the old paper was slowest, and reading the tables rather than the sentence is what tells you that.

Notice the unfiltered column too — 500 on both papers, roughly a stop faster than any filtered grade, because nothing is being absorbed. That is the column Part VII used for a paper negative in a camera, where the subject is lit by whatever is lighting it and no filter is in the path. That page also flagged the limit of the transfer, and the flag stands: ILFORD measured these figures for the paper as a printing material, and reading the same exposure scale as governing the paper in a camera is this course’s own inference, sound but not ILFORD’s claim.

What the developer does to the curve

Development moves a paper curve, but not the way it moves a film curve, and the difference trips people up.

Development time mostly moves maximum black and the toe, not the slope. A paper is designed to be developed to completion: ILFORD’s own processing summary gives one minute in MULTIGRADE developer at 1+9 at 20 °C, and warns in plain words that prints developed for shorter times may be underdeveloped and lacking in contrast and density. The whole scale is meant to arrive, and it arrives at about a minute.

✕Pulling a print out early is not a contrast control

The darkroom folklore is that snatching a print from the developer early holds the highlights back. The sensitometry says otherwise, and so does the maker.

What short development actually does is fail to complete the shadows, so maximum black is lost and the print goes muddy and slightly flat, while the highlights — which were barely exposed and are on the long flat toe — move hardly at all. You have not reduced contrast in any useful sense; you have thrown away the bottom of the scale. The print looks softer because it has no black in it, which is not the same thing as having a longer exposure range.

The honest controls are the ones with sensitometry behind them: change the grade to change the exposure scale, change the exposure to move where the tones land on it, and control locally with burning and dodging, which Part XIX owns. A stop bath belongs in the sequence for the same reason — ILFORD recommend one specifically because it stops development immediately, which is what makes a development time a controlled variable rather than a rough one.

Exhaustion lowers Dmax and raises the effective speed requirement. A developer that has processed too many prints reaches a lower maximum black in the same time, and the print needs more exposure to look the same. That is drift, not a control: it is what process control exists to catch, and it is why a session’s first and last prints should be compared before anything else is blamed.

Temperature and dilution move the time, not the destination. ILFORD specify 20 °C ± 1 and note that high temperatures reduce solution life considerably; their MULTIGRADE developer runs one minute at 1+9 and 1½ at 1+14. The published pH of the working solution is 10.45 to 10.55 at 1+9. What the developer formula does to the image colour of a chlorobromide paper is a different subject and belongs to the printing parts, along with the formulas themselves — this page teaches no paper-developer formula, because a formula belongs in the formulary and its practice in Part XVIII.

Dry-down, and what the course does not know

A wet print judged under a bright darkroom lamp looks lighter and more open than the same print dry, on a wall, in ordinary room light. Highlights close up, the deepest tones separate less, and a print matched wet is often a shade too dark dry.

The course has found no manufacturer figure for the magnitude of this on a developing-out silver gelatin paper. It searched, and what it found instead is that dry-down is documented and taken seriously in the printing-out and alternative processes: suppliers’ instructions for gold-toned printing-out paper warn that prints will have a significant amount of dry-down and will get darker as they dry, and similar warnings attach to palladium and to albumen. Those are real Tier-1 statements about different materials, and this course will not transfer a number across from them.

So dry-down is presented here as it honestly stands: a practitioner observation with a mechanism, no sourced magnitude for silver gelatin, and a measurement you can make yourself.

🧪Measure your own dry-down in one session

Two sheets, one exposure. Print a step wedge or a grey scale onto a sheet at your standard exposure and development. Cut it in half while wet.

Judge one half wet under your usual viewing lamp and mark the step you call “just off white” and the step you call “just off black”. Dry the other half fully, view it in the light the print will actually be seen in, and mark the same two steps.

The difference, in steps, times your wedge’s increment, is your dry-down expressed as a density — for your paper, your developer, your drying method and your viewing light, which are the four things a published figure could not have known. Most printers find it is a small fraction of a stop and that it costs more at the highlight end than the shadow end. Record it as a number and apply it as an exposure correction, rather than trying to remember to “print a little lighter”.

The mechanism is worth stating even without a number. The wet emulsion is a swollen gelatin layer with a refractive index close to water’s, and light entering it is scattered and returned differently than it is from the same layer dried and collapsed against the base. It is an optical change in the layer, not a chemical change in the silver, which is why it is reversible by wetting and why it depends on the surface and the drying method.

Reading a paper curve to place a print

Now put the two curves together, which is the whole practical use of this lesson.

Your negative has a measured density range — say 1.05 between the important shadow and the important highlight. The paper’s exposure scale must be about that: 105, nearest to the published 110, which on the current MULTIGRADE RC DELUXE is filter 1.

Exposure then decides where the negative sits on that axis, and grade decides how long the axis is. Those are the two controls and they are nearly independent, which is what the speed matching across filters 00 to 3 was designed to give you: change grade without changing exposure, change exposure without changing grade.

A test strip is a crude exposure series and should be understood as one. A strip given 4, 8, 16 and 32 seconds in bands is a four-point sample of the paper’s characteristic curve, in one-stop steps, through the one negative you care about. It tells you where the negative’s densities fall on the paper’s axis. What it cannot tell you is the shape of that axis, because four points do not make a curve, and it cannot tell you about the grade at all — for that you need to look at what happened to both ends at once. A strip whose highlights are right and whose shadows are grey at every band is a grade problem and no exposure will fix it.

Two rules follow, and they are the ones printers actually use. Set the black first, because maximum black is a fixed reference the paper gives you and the eye judges everything else against it. Then look at the highlight: if it is right, the grade is right; if it is empty when the black is right, the paper’s scale is too short and you need a softer grade; if it is grey when the black is right, the scale is too long and you need a harder one.

Safelight fog is a sensitometric shift

A safelight is not a neutral part of the room. 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 “safe” is relative. Kodak put the physics behind it 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 always be minimised. ILFORD’s own recommendation for MULTIGRADE RC is no more than four minutes of direct illumination at not less than 1.2 m.

What fog does to the curve is specific and it is worse than it sounds. A small uniform exposure added everywhere lifts the bottom of the curve, where the slope is nearly zero — so it raises the base density and it compresses the highlights, which are the tones sitting on that flat foot. The mid-tones and shadows barely move, because there the curve is steep and a small added exposure is negligible beside what they already received.

So the symptom of safelight fog is not a grey print. It is a print whose whites are not white and whose highlight separation has quietly gone, with everything else looking normal — which is exactly the failure that is hardest to see, because there is nothing to compare against in the room. The measurement that finds it is a fog test, and Part XVI builds the darkroom and runs it properly.

Recap

• Reflection density is measured on light returned rather than transmitted, needs a geometry that keeps the specular glint out, and is standardised by a different part of the ISO 5 series than transmission density is. The course states its own convention and quotes no standard.
• A paper’s scale is short. ILFORD publish a maximum density of 2.15 for the current MULTIGRADE RC DELUXE and 2.05 for the one before it, so the print is the narrowest link in the whole chain.
• The curve has a long flat toe, a short steep middle and a hard ceiling. Highlights live on the toe; maximum black is a ceiling set by the material and the surface, not a taper.
• ISO(R) is the log exposure range times 100, published by ILFORD to ISO 6846:1992, and matching it to the negative’s density range is what choosing a grade is.
• A grade number is a designation; a range figure is a measurement. ILFORD’s own redesign moved the low to mid grades up to a grade harder, and the range figures record it where the numbers do not.
• Variable contrast is three emulsions with the same blue speed and different green speeds, stacked by blue light into a narrow range and staggered by green light into a wide one. Magenta is hard, yellow is soft.
• Paper speed is not film speed — ILFORD put MULTIGRADE RC at an equivalent film ISO of 3 to 6 — and it is constant across filters 00 to 3½ by design, with the step above 3½ much reduced on the current paper.
• Short development is not a contrast control: it loses maximum black rather than shortening the exposure scale.
• Dry-down has a mechanism and no sourced magnitude for silver gelatin, so the course treats it as a quantity you measure on your own paper.
• Safelight fog lifts the foot of the curve, which raises base density and compresses highlights while leaving mid-tones alone.

Check your understanding

Question 1. Your negative measures a density range of 1.45 between the important shadow and the important highlight. Using ILFORD published range figures for the current MULTIGRADE RC DELUXE (160, 130, 110, 90, 70, 60, 50 for filters 00 to 5), which filter should you try, and what happens if you use filter 3 instead?

• Filter 00; with filter 3 the print would be too light overall
• Filter 00, whose range figure of 160 is nearest to 145; with filter 3, whose range is only 0.70, the paper cannot hold 1.45 of negative density and one end or the other is lost — blocked highlights, empty shadows, or both
• Filter 2, because 90 is the middle of the range
• Filter 5, because a long density range needs a hard paper

Show the answer and why

Answer: Filter 00, whose range figure of 160 is nearest to 145; with filter 3, whose range is only 0.70, the paper cannot hold 1.45 of negative density and one end or the other is lost — blocked highlights, empty shadows, or both

Multiply the density range by 100 and take the nearest published figure: 145 is nearest to 160, so filter 00. The last option inverts the relation and it is the commonest error: a hard paper has a SHORT exposure scale, so it suits a short negative density range. With filter 3 the paper accepts only 0.70 of negative density, and the 1.45 the negative offers has to fit inside it somehow; whichever end you place correctly with the exposure, the other end runs off the paper. Note also that the arithmetic assumes the density range as projected on the baseboard rather than as read on a light box, which is ILFORD own qualification and includes your enlarger flare and its Callier effect.

Question 2. Why does exposing a variable-contrast paper with green light give lower contrast than exposing it with blue?

• Green light is less energetic, so it produces less silver
• The paper is a mixture of three emulsions of the same contrast and the same blue speed but different green speeds; blue light makes them respond together over a narrow exposure range, while green light makes them respond in sequence over a much wider one
• The green-sensitive emulsion has a lower inherent contrast than the blue-sensitive one
• Green light is scattered more by the paper base

Show the answer and why

Answer: The paper is a mixture of three emulsions of the same contrast and the same blue speed but different green speeds; blue light makes them respond together over a narrow exposure range, while green light makes them respond in sequence over a much wider one

ILFORD account of MULTIGRADE is explicit that all three parts of the mixed emulsion have the same contrast and the same speed to blue light, and differ only in how much green sensitising dye they carry. Under blue light three identical curves lie on top of one another, so their sum reaches maximum black over the same narrow exposure range one of them would: high contrast. Under green light they are staggered along the exposure axis by their different green speeds, so the sum climbs to the same maximum black over a much longer run: low contrast. The third option is the tempting wrong answer, and it is wrong because the emulsions are stated to have the same inherent contrast; the contrast difference is made by the stagger, not by the emulsions.

Question 3. A printer pulls prints from the developer at 40 seconds instead of the recommended 60, believing this holds the highlights back. What is actually happening?

• The highlights are held back and contrast is reduced, as intended
• The shadows fail to complete, so maximum black is lost and the print reads flat and muddy, while the highlights sit on the flat toe and barely move at all
• Nothing changes; development time has no effect on a paper
• The paper speed increases, so the print is darker

Show the answer and why

Answer: The shadows fail to complete, so maximum black is lost and the print reads flat and muddy, while the highlights sit on the flat toe and barely move at all

A printing paper is designed to be developed to completion, and ILFORD say plainly that prints developed for shorter times may be underdeveloped and lacking in contrast and density. The tones that are still building at 40 seconds are the heavily exposed ones at the top of the scale, so cutting the time removes the bottom of the tonal scale rather than the top. The highlights received very little exposure, sit on the long flat toe, and were finished almost immediately. The print looks softer only because it now has no black in it, which is a loss of range and not a change of the exposure scale. The controls with sensitometry behind them are the grade, which changes the exposure scale, and the exposure, which decides where the negative sits on it.

Question 4. Safelight fog has raised the paper base density slightly. Which part of the print suffers most, and why?

• The shadows, because they receive the most total light
• The highlights, because a small uniform added exposure matters most where the curve is nearly flat and the tones are separated by very little density
• The mid-tones, because they are on the steepest part of the curve
• All tones equally, because the fog is uniform

Show the answer and why

Answer: The highlights, because a small uniform added exposure matters most where the curve is nearly flat and the tones are separated by very little density

The fog exposure is uniform, but its effect is not, because the curve turns it into density at a rate that depends on where you are on the curve. On the long flat toe where the highlights sit, the tones are separated by only hundredths of density, so a small lift both raises the white point and swallows the separation between the lightest tones. Further up, where the curve is steep, the same added exposure is negligible against what those tones already received. The practical consequence is the awkward one: the failure shows as whites that are not white and highlight detail that has quietly gone, with the rest of the print looking normal and nothing in the room to compare it against. Kodak explanation is that emulsion sensitivity does not end abruptly at a wavelength, so a recommended safelight is a compromise rather than a guarantee, and exposure to it should be minimised.

Question 5. ILFORD publish an ISO Speed (P) of 240 for filters 00 to 3 on the current MULTIGRADE RC DELUXE, 220 for filters 4 and 5, and 500 unfiltered. What does the pattern tell a printer to do?

• Recalculate exposure at every change of grade
• Change grade between 00 and 3 and a half without touching the timer, make only a fine adjustment above that, and expect roughly a stop more speed if printing unfiltered
• Always use filter 2, since it is the standard
• Use the unfiltered setting for the most contrast

Show the answer and why

Answer: Change grade between 00 and 3 and a half without touching the timer, make only a fine adjustment above that, and expect roughly a stop more speed if printing unfiltered

Constant speed across the soft and middle grades is exactly what speed-matched filters are for: it makes grade and exposure two nearly independent controls, so a grade change is a test of contrast alone. The residual step above filter 3 and a half is the remnant of a much larger one, and ILFORD state that the current paper no longer has the large step change the previous generation had between grade 3.5 and grade 4, where the published speed fell from 200 to 100 — a full stop that caught out anyone who forgot it. The unfiltered column is faster because nothing is being absorbed, and its contrast is whatever the enlarger lamp own blue-to-green balance happens to give, which is why it appears in the range table at a middling figure rather than at either extreme.

Question 6. Why does this page state a maximum density of 2.15 for one specific paper rather than giving a general figure for glossy resin-coated papers?

• Because 2.15 is the maximum any paper can achieve
• Because 2.15 is the figure ILFORD publish for that named product, and the course prints a number only where it has a source for that number rather than generalising across products and surfaces
• Because fibre papers have no maximum density
• Because reflection density cannot be measured above 2.2

Show the answer and why

Answer: Because 2.15 is the figure ILFORD publish for that named product, and the course prints a number only where it has a source for that number rather than generalising across products and surfaces

The corpus holds exactly one manufacturer document that publishes a maximum density figure for a silver gelatin paper, and it publishes two: 2.15 for the current MULTIGRADE RC DELUXE and 2.05 for the MULTIGRADE IV RC DELUXE it replaced, on the same base. Those are the numbers the course may print, attached to the products they describe. A general figure for glossy resin-coated papers would be an invention dressed as a fact, and it would be a poor one, because maximum black depends on the surface as much as on the emulsion — a matt sheet cannot reach the black a glossy one reaches from the same silver, since its own texture scatters light back before it can be absorbed.

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