Variable-Contrast Papers: Three Emulsions, One Sheet

One box of paper, seven grades of contrast in half-grade steps, and the only thing that changes between them is the colour of the light you print with. That is the trick, and it is worth knowing exactly how it is done, because almost every printing decision after this page — which filter to start at, whether a test strip made at one grade is still valid at another, whether a grade number carries from your enlarger to somebody else’s, whether green light from a safelight matters more than it looks — falls out of the mechanism rather than out of practice.

Part XIII owns the sensitometry of it: what a grade is on an exposure axis, what ISO(R) means and how a range figure is matched to a negative. That is not re-derived here. This page is about the material, the filters and the lamp, and about the three of them being one system.

Graded paper first, because it defines the problem

A graded paper carries one emulsion of one contrast. Its exposure scale is what it is, and to change contrast you change the packet.

Getty describe how the grades were made: graded papers “were manufactured with silver halide particles of different sizes”, uniform particle sizes giving a high-contrast paper and mixed sizes a lower-contrast one. That is worth pausing on, because it says a grade is a material property. A uniform emulsion has one threshold, so every crystal turns over together and the whole scale is covered in a short span of exposure — high contrast. A mixed emulsion has a spread of thresholds, so the crystals turn over in sequence across a long span — low contrast.

A manufacturer wanting five or six grades therefore had to design, make, coat, box and stock five or six different emulsions, and a printer had to buy and store all of them. ILFORD’s surviving graded paper shows both the cost and the virtue:

ILFOBROM GALERIE FB	Grade 1	Grade 2	Grade 3	Grade 4
ISO(R)	130	110	90	70
ISO speed (P)	400	400	400	400

Four equally spaced grades in glossy, three in matt, and — the interesting column — one speed for all of them. No filter is in the light path, so nothing is being absorbed, so the exposure does not change when the grade does. ILFORD’s instruction for using it on a colour enlarger is a single line: expose with white light, without filtration.

What a graded paper still does better. One well-behaved emulsion per sheet, with no filter between lamp and paper to scatter, fade or collect fingerprints; no speed step anywhere in the range; and a curve shape that is the emulsion’s own rather than the sum of three. The costs are inventory, half-grades you cannot have, and no way to give two parts of one print two different contrasts.

Where the idea came from, and how long it took

The Getty atlas gives the sequence, and it is a long one for an idea this simple.

When	What	Source’s wording
1911	Rudolph Fisher patents, in Germany, a black-and-white paper with variable response to different colours of light	“This paper was never marketed”
1940	The US Defender Company introduces the first variable-contrast paper	Getty print the name as Varigram
about 1940	Ilford Ltd introduce the first Multigrade paper in England	Commercially available only in 1950 because of the war, and exported to the United States from 1956
Spring 1957	Kodak introduce Polycontrast
1990s	Variable-contrast papers finally become the dominant black-and-white papers	Graded and VC sold side by side for fifty years before that

⁇Varigram or Varigam?

The Getty atlas prints the Defender product’s name as Varigram. The name that circulates almost everywhere else — in printers’ accounts, in secondary histories and in collectors’ listings — is Varigam.

The course has not found a Defender or DuPont publication, patent or advertisement in its corpus to settle which is right, and a conservation atlas is a Tier 1 source while the circulating spelling has no source at all. What the course states: the atlas prints Varigram; the name usually seen elsewhere is Varigam; the discrepancy is unresolved here and should be settled from the maker’s own literature if that is ever found. Nothing in the chemistry depends on it.

The date does not appear to be in dispute — 1940, in both accounts — nor does the priority, which Getty give to Defender by a short head over Ilford’s first Multigrade, with Ilford’s product delayed to market by the war.

Two things in that table are worth carrying forward. Fisher’s patent is twenty-nine years before the first product, which says the hard part was never the principle but the emulsion-making: three components that match in contrast and blue speed and differ only in green sensitisation are a manufacturing problem, not an idea. And VC papers took another fifty years to displace graded ones, which is a reminder that the graded paper’s virtues in the previous section were real and were believed in by working printers for two generations.

The variable-contrast principle

ILFORD’s own account is worth following exactly, because everything else follows from it.

MULTIGRADE papers are coated with an emulsion which is a mixture of three separate emulsions. Each emulsion is a basic blue sensitive emulsion to which is added different amounts of green sensitising dye. […] All parts of the emulsion have the same contrast. They also all have the same speed to blue light, but naturally, the part of the emulsion with only a small amount of green sensitising dye has a low speed to green light.

Three sentences, and three things to notice.

Three, not two. ILFORD say three for MULTIGRADE and the Getty atlas, describing the class rather than one product, says “a mixture of two or three silver halide emulsions”. Any account of a MULTIGRADE paper that says two is not following the maker.

Same contrast, same blue speed. This is the sentence most explanations get wrong. The components are not a soft emulsion and a hard emulsion mixed in varying proportion. They are three emulsions of identical inherent contrast and identical blue speed, differing in one property only: how much green sensitising dye each carries, and therefore how fast each is to green.

The dye is the whole variable. Spectral sensitisation is Part IV’s subject and Part V coats a dye by hand, so the mechanism — a dye adsorbed to the crystal surface absorbing a photon the halide could not and handing an electron to the conduction band — is already established. Here it is used as a speed control rather than as a way of reaching a new colour.

Why that gives contrast and not merely exposure

⚗Three curves, added twice

Take three components with identical curves, and ask what their sum does under two different lights.

Blue. All three have the same blue speed, so all three curves sit on top of one another on the exposure axis. Adding three identical curves gives a curve of the same shape: three times the density at every exposure, but over the same span. Normalise to the one maximum black the paper actually has, and the sum covers the whole scale in a narrow band of exposure. Narrow exposure range is high contrast.

Green. The three have very different green speeds, so their curves are staggered along the exposure axis. The most heavily dyed component answers first and does its whole rise; the middle one begins as the first is finishing; the least dyed one needs much more light still. The sum climbs to the same maximum black over a span perhaps three times as long. Wide exposure range is low contrast.

Same silver, same emulsions, same maximum black. The only thing that changed was whether the three curves were stacked or staggered, and that was decided by the colour of the light.

Every mixture of blue and green in between gives a partial stagger, which is why a variable-contrast paper gives a continuum of grades rather than a set of steps, and why a maker can sell twelve filters and a head that resolves tenths.

The filters follow in one line each. Magenta absorbs green and transmits blue, so magenta is the hard direction. Yellow absorbs blue and transmits green, so yellow is the soft direction. Foma say the same thing about their own papers in the same words, from the paper’s side: expose only the blue-sensitised part, under magenta, and contrast increases; expose the green-sensitised part, under yellow, and it decreases.

What the grades look like on one exposure axis

• Filter 00 — ISO(R) 160
• Filter 1 — ISO(R) 110
• Filter 2 — ISO(R) 90
• Filter 3 — ISO(R) 70
• Filter 5 — ISO(R) 50

Drawn, not measured. The horizontal span of each curve is ILFORD's published ISO(R) figure for MULTIGRADE RC DELUXE divided by 100; the base, the maximum and the shape between them are drawn to teach and were not read off a material. In practice filters 4 and 5 also need between half a stop and a stop more exposure, which would shift those curves to the right. 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.

Filtration in practice: three routes to the same colour

Three ways to set the colour of the printing light

1. 1Filters above the lensin the enlarger’s filter drawer, in the light path before the negative — a scratch or a fingerprint is out of focus and does not print
2. 2Filters below the lensin a holder under the lens — convenient and quick, but the filter is now in the image-forming path, so its cleanliness is part of the image
3. 3Dialled into the heada dichroic colour head, or a purpose-built variable-contrast head, with yellow and magenta set as numbers rather than swapped as pieces

ILFORD supply their filters for either position and warn that dust, rough handling and finger marks on a filter reduce print contrast and degrade the image.

Hand filters. Twelve MULTIGRADE filters, numbered 00 to 5 in half steps, the lowest number the softest. They may be used above or below the lens, or cut to fit a filter drawer.

Dichroic colour heads. A colour enlarger already has yellow and magenta filtration on dials, so it can do this job — with a caveat ILFORD state themselves: the maximum contrast will be slightly lower, because a colour head’s filters are optimised for colour paper rather than for variable-contrast paper. Their published settings are explicitly “a guide”, because “individual enlargers vary”, and “the actual filtration for a particular enlarger must be determined by trial”.

Here is one row of that table, which makes the point better than the whole of it:

MULTIGRADE filter	Durst (max 170M)	Durst (max 130M)	Kodak	Meopta
00	150Y	120Y	199Y	150Y
2	0	0	0	0
3	45M	30M	25M	40M
5	170M	130M	199M	—

Four heads, one paper, one grade — and four different numbers, one of which does not exist at all, because that head cannot reach grade 5.

!A grade number is not a measurement

Read the table above and the conclusion is unavoidable. The correspondence between a filter number and a contrast grade is a property of the paper, the filter and the light source together. Change any one of the three and the correspondence moves.

Foma prove it on a single sheet of paper. FOMABROM VARIANT is published with two technical-data tables, one for ILFORD MULTIGRADE filters and one for Foma’s own Variant filters, and the same paper gives different range figures, different speeds and different exposure factors under each:

	ILFORD filter 3	Foma filter 2×M1
What Foma call the grade	normal	normal
ISO(R)	70	80
ISO speed (P)	200	240
Lengthening factor	2.4	2.1

Both of those rows are the grade Foma themselves name “normal” on that paper — and the two disagree about the paper’s exposure scale by ten points of range, about its speed by a fifth, and about the exposure factor. The control is in the same two tables: the unfiltered row reads R100 at ISO P500 under both headings, because with no filter in the path there is nothing to disagree about. The paper has not changed. Only what is in front of it has.

One emulsion, one maker, one printed sheet, two filter sets, two answers. What is transferable is the range figure, because it is a measurement of the paper’s exposure scale under a stated system — which is exactly why Part XIII treats ISO(R) as the quantitative form of “grade” and treats the grade number as a maker’s designation.

Speed matching, and the step near the top

ILFORD’s filters are speed-matched over most of their range, and this is a design decision with a visible consequence. Their current leaflet: grades 00 to 3½ are speed matched, and “little or no adjustment to exposure time is necessary when changing between these grades”; grades 4 to 5 “will generally require more exposure … in practice this will be between 0.5 – 1 stop depending on the product”.

Why the step exists at all is worth being careful about. ILFORD publish the effect and not the mechanism. Part XIII offered the course’s own reading, marked as an inference: the hard grades are made by cutting the green out with magenta, and past about filter 3½ the filter is absorbing a large share of the lamp’s total output, so less light arrives. The emulsion is no less sensitive. That reading is consistent with the direction of the change and with the unfiltered column being the fastest of all, and no manufacturer statement confirming it has been found.

🧮Two published numbers that do not agree, and what to do about it

ILFORD’s ISO speed table for MULTIGRADE RC DELUXE gives P240 at filters 00 to 3 and P220 at filters 4 and 5. That is a ratio of 240 ÷ 220 = 1.09, or 0.13 of a stop.

Their filter leaflet says grades 4 to 5 need 0.5 to 1 stop more exposure.

Both are ILFORD’s own figures and they differ by a factor of five or more. They are not measuring the same thing: an ISO speed is defined at one density point under a specified procedure, while a practical filter factor is what a printer must do to keep a whole print looking the same. A speed point near the toe can move very little while the exposure needed for the print moves a great deal.

What to do about it. Neither number, and both. Make a test strip at your working grade, change to grade 4½ or 5, and measure your own factor for your own paper, filters and lamp. Then write it on the inside of the enlarger’s filter drawer, where you will find it. Part XVII’s f-stop timing gives the arithmetic for applying it in stops rather than seconds, which is the only sensible unit for it.

The practical corollary is the one every printer meets: a test strip made at one grade is not valid at another unless you know the factor between them, and across 00 to 3½ on ILFORD’s system that factor is about one, which is the whole point of speed matching.

Choosing a starting filtration from a measured negative

This is the quantitative use of everything above, and Part XIII already established the method. Measure the effective density range of the projected image, multiply by 100, and look up the nearest ISO(R) figure. ILFORD’s own worked example: a negative with an effective density range of 1.32 log exposure units gives 132, so choose the filter whose range figure is 130 — which on MULTIGRADE RC DELUXE is filter 0, and on MULTIGRADE IV RC DELUXE was filter 1.

Two warnings travel with it.

The range figure belongs to the product, not to the family. The current RC DELUXE and the MULTIGRADE IV it replaced differ by up to a grade at the same filter number, and ILFORD say so: in the redesign the low to mid grades were offset up to one grade harder.

A range figure is not an exposure. It tells you which grade will accept your negative’s whole scale. It says nothing about where on the paper’s scale the print will sit, which is exposure, and which is the experiment’s business.

The source spectrum is the other half of the system

A filter can only subtract. If the lamp has little green in it, a yellow filter has little to work with and the soft grades collapse; if it has little blue, magenta has nothing to save and the hard grades collapse. The grade range available is a property of the source as much as of the paper.

Where the three components respond, and what four sources put there

• All three components: blue response (340–500 nm) — same speed in all three — this is the hard direction
• Green-dyed response (480–555 nm) — three different speeds here — this is the soft direction
• Blue LED channel (450–485 nm) — Cree XP-E2 blue dominant-wavelength bins, 465–485 nm; royal blue 450–465 nm
• Green LED channel (520–535 nm) — Cree XP-E2 green dominant-wavelength bins
• Safelight region (575–640 nm) — makers publish cut-offs from 575 nm to 625 nm depending on paper

Inferred, not measured. No manufacturer in the course's corpus publishes spectral sensitivity curves for the individual components of a variable-contrast paper; ILFORD print a wedge spectrogram with no numbered sensitivity axis. The response bands are drawn from ILFORD's and Foma's own descriptions and from their published safelight cut-offs. The LED bands are read from the Cree XP-E2 datasheet's dominant-wavelength bins. The coloured strip approximates where the visible spectrum falls and is a reading aid only; the wavelengths in the labels carry the information. The bands and curves are drawn to show the relationship, not measured.

Tungsten and tungsten-halogen. What every maker designs for, and what every published filter number assumes. Continuous, with plenty of green and enough blue.

Cold cathode. ILFORD distinguish carefully between cold cathode heads designed for variable-contrast papers, which can give a full contrast range, and everything else, which “may give a reduced contrast range”. Even the designed ones may space the grades unevenly. Their Aristo W45 data is the honest version of this: a full range is available, “but the grade intervals are bunched towards the hard contrast end”, and a whole matrix of exposure factors is needed to move between grades — the factor from filter 2 to filter 3 on that head is 0.74, which is a shorter exposure for a harder grade, the reverse of what a tungsten head does.

Dichroic colour heads. Covered above: usable, slightly short of the hardest grade, and needing their own calibration.

LED. ILFORD’s current RC sheet accepts “LED exposing heads designed for variable contrast papers” — and the qualifier is doing work.

⁇What a white LED can and cannot do, and why the course publishes no claim

Part XVI built a single-channel white LED head and deliberately deferred three questions to this part. They can now be answered, two of them from published data and one not at all.

Can a phosphor-converted white LED reach the full grade range on a named paper? Unknown, and this course will not say. A white LED is a blue die plus a phosphor, so its spectrum has a blue peak and a broad phosphor hump — a fixed blue-to-green ratio you can only subtract from with filters, exactly as with a tungsten lamp but from a different starting mixture. No manufacturer statement in the corpus covers it, and the course has not measured it. ILFORD’s phrasing, “designed for variable contrast papers”, is a qualifier and not an endorsement of white LEDs generally. Measure it: the test is a step wedge exposed at filter 00 and at filter 5 through the head, the two ISO(R) figures compared with the maker’s published pair.

How finely must the blue-to-green ratio be settable? Half a grade is the working granularity the whole industry sells: twelve filters in half steps. ILFORD’s own professional head went finer, giving contrast control in tenth-grade steps over grades 0 to 5, so a tenth is achievable and was thought worth productising. The requirement is that the setting be repeatable, not that it be infinitely fine — an irreproducible tenth is worse than a reliable half.

Must the two channels be speed-matched? No, if each channel is timed separately. Speed matching is a convenience that lets a printer change grade without recomputing exposure, and ILFORD achieved it with filter densities rather than with anything in the paper. A two-channel head gives each channel its own exposure time, so the ratio and the total are set independently, and the printer computes in stops as Part XVII teaches. What the head does need is stable output per channel, which is what Part XVI’s constant-current driver and heatsinking are for — and the green emitter is the harder half, since the Cree part Part XVI works through has a thermal resistance of 9.0 °C/W in green against 5.7 in blue.

Does a third channel earn its place? No, on the published evidence. The paper has three emulsion components but only two spectral handles, because all three share one blue speed and differ only in green. ILFORD’s entire system is two-variable — yellow and magenta — and no maker in the corpus publishes a component with a third distinct sensitisation. Build two channels. Part XVI’s advice to drill and wire the second position stands, and the third one is not needed.

One more source effect, which is not spectral at all. ILFORD state that a condenser enlarger gives about an extra grade of contrast compared with a diffuser, that the difference depends on the amount of silver left in the negative, and that it nearly vanishes for very pale flat negatives and for dye images such as XP2 SUPER. That is the Callier effect, which Part XIII owns and which this page does not re-derive — but it means a grade number carried between a condenser and a diffuser enlarger is wrong by about a grade before any filter is considered.

Split filtration, as a mechanism

If the hard image and the soft image are made by two different lights, they can be made by two different exposures. Give the paper a timed exposure through a hard filter and a second timed exposure through a soft one, and the two sum on the sheet.

The mechanism is sound and follows directly from ILFORD’s account: the blue exposure addresses all three components together and lays in the high-contrast image; the green exposure addresses the components in sequence and lays in the long-scale one. Because the paper integrates whatever it receives, the printer has two independent controls — the hard time and the soft time — instead of one filter number, and each can be dodged and burned separately.

⁇Where the evidence for split-grade printing runs out

What is established: two exposures through two filters sum on the paper, because a photographic emulsion integrates exposure; and each can be given its own dodging and burning, which is a real and uncontroversial gain in control.

What is not established in this course’s corpus: that a split pair produces a curve of a different shape from the single grade of equivalent slope, rather than merely reaching the same place by a different route. That is the specific claim the course has looked for a source for and not found. No manufacturer publishes measured curves for split exposures.

The course’s position, following its ruling on practices with thin evidence: split-grade printing is taught as a control method and not as a curve-shape claim. Part XIX practises it. If the shape claim is to be made at all, it will be made from a measurement in this part’s own experiment framework and published as the course’s own data, labelled as such.

The safelight consequence, which is worse than it looks

A green sensitising dye does not know the difference between green from the enlarger and green from the lamp you are working under.

Part XVI established what follows, and it is worth restating in one sentence because it is easy to miss: green light on a variable-contrast paper does not merely fog it, it softens it, because a green exposure is by construction a low-contrast exposure. A leaking lamp adds veiling density and pulls the grade towards the soft end, so a printer chasing lost contrast adds magenta, loses speed, extends the exposure, and gives the leak longer to work.

The makers’ own numbers show they treat variable-contrast papers as the harder case. Foma ask for 575 nm and above on the graded FOMABROM and 625 nm on the variable-contrast FOMABROM VARIANT, which they describe as orthochromatically sensitised — a 50 nm difference between two papers from one factory. ILFORD ask for a cut-off no lower than 580 nm on MULTIGRADE FB CLASSIC.

The rule that follows is procedural: run Part XVI’s fog test with the paper you actually print on. A safe working time measured on a graded paper does not transfer to a variable-contrast one, and a time measured on one maker’s variable-contrast paper does not transfer to another’s.

Contrast is not only filtration

Filtration is the largest and most controllable handle, and it is not the only one. Five others move print contrast, and they differ in whether they are legitimate controls or symptoms of something wrong.

Handle	Direction	Is it a control?
Filtration	both, continuously	Yes — the designed control
Developer choice	modest, both ways	Yes, but it changes tone and speed with it
Dilution	usually softer when weaker	Partly — ILFORD offer 1+9 and 1+14 as “greater control and economy”
Development time	very little on paper	No. ILFORD state development may be extended to six minutes “without any noticeable change in contrast or fog”
Flashing and pre-exposure	softer, in the highlights only	Yes — a real control, practised in Part XIX
Green safelight leakage	softer, and foggier	No. This is the fault above

The fourth row is the one that separates paper from film, and it is the whole argument of the next page: a film is pulled from the developer at a chosen contrast, and a print is developed to completion, so development time is not a contrast control on paper. Anyone using it as one is either underdeveloping — which shows as weak, mottled blacks rather than a lighter print — or has reached the end of the plateau.

Flashing and pre-exposure are the interesting exception, and they belong to Part XIX rather than here. The principle is that a small uniform exposure, below the paper’s threshold on its own, lifts the whole sheet to the foot of its curve, so that the faintest image exposures now land on a responsive part of the scale instead of on the toe. It compresses the highlights and leaves the shadows alone, which is a change in curve shape that filtration cannot make. No manufacturer in the corpus publishes guidance on it, so it is practised and measured rather than looked up.

Recap

• A graded paper is one emulsion. Getty attribute its grade to crystal-size uniformity: uniform sizes give high contrast, mixed sizes low. ILFOBROM GALERIE FB carries four grades at one speed, R130 to R70, exposed with white light and no filter.
• A MULTIGRADE paper is three emulsions, of the same contrast and the same blue speed, carrying different amounts of green sensitising dye. ILFORD say so; Getty say two or three for the class.
• Blue stacks the curves and green staggers them. Stacked is narrow and hard; staggered is wide and soft. Magenta transmits blue and is the hard direction; yellow transmits green and is the soft one.
• A grade number is a property of paper, filter and lamp together. Foma print two different tables for two filter sets on one sheet of paper. ISO(R) is the transferable number.
• Filters 00 to 3½ are speed matched; 4 and 5 are not. ILFORD’s own two published figures for that step disagree by a factor of five, so measure your own.
• The source is half the system. Cold cathode may bunch the grades, colour heads fall slightly short of grade 5, condensers add about a grade over diffusers, and no claim is published here about what a white LED can reach — that is a measurement, and the test is stated.
• Two LED channels are enough — blue at 450 to 485 nm and green at 520 to 535 nm on the datasheet Part XVI works from — because the paper has three components but only two spectral handles.
• Green safelight leakage fogs and softens. Test with the paper you print on.
• Development time is not a contrast control on paper. Flashing is.

Check your understanding

Question 1. A negative measures an effective density range of 0.95 log units on the baseboard. Using ILFORD’s published ISO(R) table for MULTIGRADE RC DELUXE — 160, 130, 110, 90, 70, 60, 50 at filters 00, 0, 1, 2, 3, 4, 5 — which filter should you start at, and why that one?

• Filter 00, because the softest grade always shows the most detail
• Filter 2, because 0.95 × 100 = 95 and the nearest published range figure is 90
• Filter 5, because a short density range needs a hard grade
• Filter 0, because the paper’s unfiltered range is 90

Show the answer and why

Answer: Filter 2, because 0.95 × 100 = 95 and the nearest published range figure is 90

ILFORD’s own instruction is to multiply the effective density range by 100 and choose the nearest range figure; 95 sits between 110 and 90 and is nearer 90, which is filter 2 on this product. The reasoning behind the arithmetic is Part XIII’s: a grade is a length of exposure axis, and you want the paper whose exposure scale is about as long as the negative’s density range, so that neither end of the negative falls off the paper. Note that the same 0.95 would send you to filter 1 on the MULTIGRADE IV this paper replaced, because ILFORD shifted the low and mid grades up to a grade harder in the redesign.

Question 2. A test strip is made at filter 2 and the final print at filter 4½. What happens to the exposure, and how much?

• Nothing — ILFORD’s filters are speed matched across the whole range
• It must increase, and ILFORD’s published figures for how much disagree with each other, so you measure your own
• It must decrease, because a harder grade needs less light
• It must double exactly, on every ILFORD paper

Show the answer and why

Answer: It must increase, and ILFORD’s published figures for how much disagree with each other, so you measure your own

Speed matching runs from 00 to 3½ only. Above it, ILFORD’s filter leaflet says 0.5 to 1 stop more depending on the product, the older FB CLASSIC sheet says typically 1.5 times to double, and their ISO speed table implies only 0.13 of a stop between P240 and P220. Those are not measuring the same thing — a speed point is defined at one density, a practical factor is what keeps a whole print looking the same — and the resolution is not to pick one but to measure the factor for your own paper, filters and lamp, and to work in stops.

Question 3. Design a test that would establish whether a given white-LED enlarger head can reach grade 5 on a named variable-contrast paper. What is the control, and what is the single variable?

• Print one negative at filter 5 under the LED head and judge whether the print looks contrasty
• Expose a step wedge through the head at filter 00 and at filter 5, process to the maker’s recommendation, and compare the two measured ISO(R) figures against the maker’s published pair — the control being the same paper, filters and processing under a tungsten head
• Measure the LED’s spectrum with a spectrometer and compare it with the paper’s wedge spectrogram
• Compare the LED head with a dichroic head at grade 5 by eye

Show the answer and why

Answer: Expose a step wedge through the head at filter 00 and at filter 5, process to the maker’s recommendation, and compare the two measured ISO(R) figures against the maker’s published pair — the control being the same paper, filters and processing under a tungsten head

The claim under test is about the paper’s exposure scale, so the measurement has to be of the exposure scale: a step wedge gives it, and ISO(R) is the number. The control is the same paper, the same filters and the same processing under the source every published figure assumes, which is tungsten. The single variable is the lamp. A spectrometer reading is informative but does not answer the question, because the answer depends on the paper’s response as well as on the source; and judging one print by eye tests nothing, since there is no reference for what grade 5 should look like.

Question 4. Why is a green light leak from a safelight worse for a variable-contrast paper than an equal leak of deep red?

• Green light is more energetic than red
• Because green falls inside the sensitised response, so it both adds veiling density and, being a low-contrast exposure by construction, softens the paper
• Because green light bleaches the sensitising dye
• Because red light is absorbed by the baryta layer

Show the answer and why

Answer: Because green falls inside the sensitised response, so it both adds veiling density and, being a low-contrast exposure by construction, softens the paper

Deep red falls outside the response of every component. Green falls inside the dyed components’ response, so it exposes them — and a green exposure is the soft exposure, the one that staggers the three curves. The print therefore loses highlight separation to fog and loses contrast at the same time. The trap for the printer is that the obvious remedy, adding magenta, lengthens the exposure and gives the leak longer to act. Foma’s own numbers show makers treat this as the harder case: 575 nm on their graded paper, 625 nm on their variable-contrast one.

Question 5. A printer says their negative "prints at grade 3" and gives a friend the number. What travels between the two darkrooms, and what does not?

• Everything travels — a grade is a standardised quantity
• Nothing travels; contrast cannot be communicated
• The negative’s effective density range travels; the filter number does not, because the filter-to-grade correspondence depends on the paper, the filter set and the source together
• The filter number travels if both printers use ILFORD paper

Show the answer and why

Answer: The negative’s effective density range travels; the filter number does not, because the filter-to-grade correspondence depends on the paper, the filter set and the source together

A measured density range is a property of the negative and is transferable; the friend can look it up against their own paper’s ISO(R) table. The filter number is not, and Foma demonstrate why on one sheet: FOMABROM VARIANT under ILFORD filters and under Foma filters gives different range figures, speeds and lengthening factors for the same named grade. Add a condenser instead of a diffuser — about a grade of difference on ILFORD’s own statement — or a colour head that cannot reach grade 5, and the number has moved again before anyone has made a mistake.

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