Grain, Speed and Resolution

Everything on the last three pages has been about one crystal. A photograph is made of several thousand million of them, and almost every choice a manufacturer makes about that population — how big, how uniform, what shape, how deep, on what base — you experience as a property of the picture: its speed, its contrast, its grain, its sharpness, and whether a street lamp at night prints as a point or as a glowing disc.

This page is where the crystal becomes the photograph.

Bigger is faster, and how far that gets you

The reason a larger crystal is faster is the least chemical fact on the page. It is geometry. A crystal becomes developable when it has absorbed enough photons to build a cluster of about four silver atoms, and how quickly it collects them depends on how much of the light crossing the emulsion it intercepts — which is its projected area, the shadow it casts. Double the edge of a cube and you quadruple its target area, so at a given illumination it reaches the threshold four times sooner.

That prediction was tested with a microscope before anyone could explain it. Bellach measured the grains of the Apollo plate at three stages of manufacture and reported them alongside the makers’ descriptions of speed:

Plate	Mean grain	Described as
Apollo, stage I	1.6 µm	very insensitive
Apollo, stage II	2.3 µm	insensitive
Apollo, stage III	3.6 µm	very sensitive

Sheppard and Mees, measuring the same way in 1907, got 1.7 µm for a Wratten Ordinary plate at 20 H&D, and for the Imperial Special Rapid at 200 H&D — ten times the speed — two distinct populations at 1.1 and 3.4 µm. Bigger crystals, faster plate: the trend was measurable a century ago and it has not changed.

!The ceiling that size alone runs into

So why is a modern film not made of enormous crystals?

Because speed and grain are bought from the same account. A developed crystal is all-or-nothing: it contributes the whole of its silver or none of it. So making the crystals bigger raises speed roughly with their projected area, but it also means that each one delivers a bigger lump of silver, and the picture is built from fewer, coarser pieces. Meanwhile covering power falls as one over particle size, so the bigger grains give less density per gram of silver, and the emulsion has to carry more silver to reach the same maximum density.

There is a second ceiling behind the first. A crystal needs only about four photons’ worth of trapped electrons. Make it enormous and most of the photons it intercepts land where no useful trap is available, or free electrons that recombine before an ion reaches them; the extra capture area is wasted. Kodak’s own double-jet patent quietly records where the practical range sits: it states that particularly good results come from emulsions in which the average grain size is less than 0.5 micrometres, and its worked example makes cubic-regular grains of about 0.2 micrometres.

The whole history of emulsion design since is an attempt to break the link between capture area and silver volume, and the section on morphology below is the answer that worked.

Size distribution: one number is not enough

The average crystal size tells you where the population sits. The spread tells you what shape the characteristic curve will have, and it is the more useful number of the two.

Think about what happens as exposure rises across a population of crystals. Each crystal has its own threshold, set mostly by its size. If every crystal is nearly the same size — a monodisperse emulsion — then they all cross their thresholds within a narrow band of exposure, and density goes from nothing to everything over a short stretch of the log-exposure axis. That is high contrast and a short scale.

If the population is broad — polydisperse — the big crystals cross first, the medium ones later and the small ones later still, so density climbs gradually over a wide stretch of exposure. That is lower contrast and a long tonal scale, and it is what a general-purpose pictorial film wants.

This is not a deduction. Manufacturers say so. BERGGER’s Pancro 400 sheet states that the film includes two photographic silver-bromide/iodide emulsions that differ in grain size in order to achieve the film’s outstanding exposure range — two populations deliberately blended, for exactly the reason above. Sheppard and Mees found the same structure by microscope in 1907 in the fast Imperial plate, and found the slow plate “practically homogeneous”. Fast, long-scale films are mixtures; slow, contrasty ones are not.

What the size distribution does to the curve

• Narrow distribution: short scale, high contrast
• Wide distribution: long scale, lower contrast

Drawn to teach, not measured. Both curves carry the same maximum density: what a wide size distribution buys is not more density but more exposure range to put it in, which is why a fast general-purpose film forgives a metering error and a lith film does not. 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.

There is a third use for the same trick, and this course meets it two pages later. ILFORD’s Multigrade papers are three emulsions blended in one coating — but there the three differ in how much green sensitising dye they carry rather than in size, so the blend is switched by the colour of the printing light instead of by its amount. Same engineering idea, different variable; the spectral sensitivity page has it in full.

Morphology: cubes, octahedra, tablets and shells

A cubic lattice does not force a cubic crystal, as the silver halides page showed: which faces survive depends on the solution the crystal grew in, and Kodak’s patent holds the pAg between 8.6 and 9.2 to get cubes rather than octahedra. Four habits matter for what a photographer sees.

Four crystal habits, and the shadow each one casts

1. 1Cube — the (1,0,0) faces; pAg 8.6 to 9.2 in Kodak's double-jet patent
2. 2Octahedron — the (1,1,1) faces; what ordinary precipitation tends to give
3. 3Tabular grain — large projected area, small volume — speed without the silver
4. 4Core-shell crystal — inner and outer regions of different composition or sensitisation
5. 5Projected area — the shadow the crystal casts: what decides how much light it catches

Habit, not scale: all four are drawn the same size on the page and a real emulsion's crystals are around 0.2 micrometres. The shading beneath each is the projected area, and the comparison between the third and the first is the argument the tabular grain was invented to make.

Cubic and octahedral crystals are compact: scale one up and its shadow and its silver content grow together, so speed and grain rise in step and you never get ahead.

Tabular crystals break that. A tablet lies flat in the coating, so it presents a large face to the light while containing only as much silver as its small thickness allows. It catches light like a big crystal and develops like a small one. Kodak’s trade name for the technology is T-GRAIN, and both T-MAX films carry it.

Core–shell crystals are built in two stages so that the inside and the outside differ — in halide composition, or in where the sensitivity centres sit. Kodak’s 1952 Davey and Knott patent is an early example of the idea used to put the latent image inside the crystal on purpose, which is the internal latent image the latent image page described.

📷The tabular-grain advantage, in Kodak's own published numbers

This is not a marketing claim; you can read it off four data sheets. Kodak publishes diffuse rms granularity for its black-and-white films, all measured the same way — at a net diffuse density of 1.00, through a 48 micrometre aperture, at 12× magnification, after development in D-76 at 20 °C.

Film	Speed	Emulsion	Diffuse rms granularity
T-MAX 100	ISO 100	T-GRAIN (tabular)	8
T-MAX 400	ISO 400	T-GRAIN (tabular)	10
TRI-X 320	ISO 320	conventional	16
TRI-X 400	ISO 400	conventional	17

Read the two ISO 400 rows against each other. Same speed; granularity 10 against 17. That is what “improved the speed-to-grain ratio” means as a number rather than a phrase, and it is the whole reason tabular grains were worth developing.

Then read down the T-MAX column: going from ISO 100 to ISO 400 within the same technology costs you 8 to 10. The trade between speed and grain has not been abolished — it has been moved. A tabular ISO 400 film is finer-grained than a conventional ISO 320 one, which would have been an absurd claim in 1970.

Speed against grain, from Kodak's published figures

• T-GRAIN tabular emulsions (T-MAX)
• Conventional emulsions (TRI-X)

Show the numbers behind this plot

Four measured points, film speed against diffuse rms granularity, taken from Kodak data sheets. The two tabular-grain T-MAX films sit on the left, at granularity 8 for ISO 100 and granularity 10 for ISO 400. The two conventional-emulsion TRI-X films sit on the right, at granularity 16 for ISO 320 and granularity 17 for ISO 400. Reading horizontally at ISO 400 shows the tabular film at granularity 10 against the conventional film at 17, which is the tabular-grain advantage at constant speed. Reading up the tabular pair shows that speed still costs grain within one technology, from 8 to 10 for two stops of speed. The four points therefore separate into two families rather than lying on one line, which is the point of the plot.

Series	Diffuse rms granularity (lower is finer)	ISO speed
T-GRAIN tabular emulsions (T-MAX)	8.00	100.00
T-GRAIN tabular emulsions (T-MAX)	10.00	400.00
Conventional emulsions (TRI-X)	16.00	320.00
Conventional emulsions (TRI-X)	17.00	400.00

Measured: the granularity figures and speeds are as published on Kodak sheets F-4016, F-4043 and F-4017, all read at a net diffuse density of 1.00 through a 48 µm aperture at 12× magnification, in D-76 at 20 °C. Two points per family is not a trend line, and the plot is not a ranking of films; it is a demonstration that speed alone does not fix grain.

Microcrystal, developed grain, and the thing you actually see

Three different objects get called “grain”, and the course keeps them apart, because confusing them makes the whole subject incoherent.

A microcrystal is one crystal of silver halide in the unexposed emulsion. In a modern film it is about 0.2 micrometres across.

A developed grain is what one microcrystal becomes: a tangle of filamentary silver, roughly the size of the crystal it replaced, occupying the space the crystal used to fill.

Grain in a print is neither of those. It is the mottled texture you see when a negative is enlarged, and it is a statistical phenomenon: a clumping of developed grains, plus the random variation in how many of them happen to lie within each small patch of the image, plus the fact that they lie at different depths in a layer and overlap when you look through it. Sheppard and Mees describe measuring the developed negative by focusing down through the layer from a top layer of sparse particles to the lowest observable ones — the image is three-dimensional, and what a print shows is its projection.

🧮Proof that you have never seen a grain

This is a two-line calculation and it settles the point permanently.

A modern microcrystal is about 0.2 µm. Enlarge a 35 mm negative to a 10 × 8 inch print and you have magnified it about 8 times; go to 16 × 12 and it is about 12 times. At 12×, that crystal becomes

0.2 µm × 12 = 2.4 µm = 0.0024 mm

A crystal, enlarged

The unaided eye resolves roughly one minute of arc, which at a comfortable viewing distance of 300 mm is about 0.09 mm. The enlarged crystal is nearly forty times too small to see.

So the texture in the print is not crystals. It cannot be. It is structure some tens of times coarser than a crystal, produced by clumping and by the statistics of overlapping grains at different depths. Kodak’s measuring aperture confirms the scale from the other direction: they read granularity through a 48 µm aperture on the negative, which at their 12× magnification is 0.58 mm on the print — six times the resolution limit of the eye, and comfortably visible. The aperture is sized to the structure that matters, and that structure is about 240 crystal-widths across.

From crystal to what the eye sees: four steps up the scale

1. 1One microcrystal, about 0.2 µm — the object the last three pages were about
2. 2One developed grain — filaments occupying about the same space
3. 3A 48 µm patch of negative — about 240 crystal-widths; Kodak's granularity aperture
4. 4The same patch at 12x on the print — 0.58 mm — visible; a single crystal would be 0.0024 mm

Drawn to teach the scale relations, not to scale within each panel. The one quantitative claim is the ratio between the panels, and it comes from the crystal size in Kodak's patent and the measuring aperture on Kodak's data sheets.

Granularity and graininess

Kodak’s own sheets keep two words apart, and so does this course.

Granularity is the measured quantity: the standard deviation of density measured through a defined aperture, at a defined density, at a defined magnification. Kodak’s diffuse rms granularity figures — 8, 10, 16, 17 in the table above — are that number, and they mean nothing without the conditions attached, which is why the sheets always print them.

Graininess is the perceived quantity: how grainy a print looks to a person. Kodak use the word freely in prose (“a slight increase in graininess”) and never attach a number to it, which is the correct treatment. It depends on the granularity, but also on the enlargement, the viewing distance, the tone of the area you are looking at, the sharpness of the surrounding detail and the observer.

The bridge between them is magnification, and it has a practical corollary worth more than the definitions: there is no such thing as a grainy film, only a grainy print. A 35 mm frame from a fast film that is unbearable at 16 × 12 is invisible in a contact print, and a large-format negative on the same emulsion never gets enlarged enough for anyone to find out.

Resolving power, sharpness and the modulation transfer function

Grain is one limit on detail. It is not the only one, and the two limits are measured differently.

Resolving power is the traditional measure: photograph a target of ruled line pairs at decreasing spacings and record the finest set you can still distinguish, in line pairs per millimetre. Kodak publish it for T-MAX 100 as 63 lines/mm and for T-MAX 400 as 50 lines/mm — and then publish a second figure for each, 200 lines/mm, from the same films.

The two figures are not a contradiction; they are the reason resolving power alone is a poor measure. Kodak states the condition beside each: the first is at a test-object contrast of 1.6:1, the second at 1000:1. A target of nearly black bars on white is far easier to resolve than one of two similar greys, so a single “lines per millimetre” figure without its contrast is uninterpretable, and the same film can honestly be quoted at 63 or at 200.

Sharpness is a different thing again: the abruptness with which density changes at an edge. A film can have high resolving power and look soft, or the reverse.

The measure that replaced both is the modulation transfer function. Instead of asking where detail disappears, it asks how much contrast survives at each spatial frequency. Kodak’s sheets plot it with spatial frequency in cycles per millimetre along the bottom and response as a percentage up the side: at low frequencies the response is 100 %, meaning a coarse pattern comes through at full contrast; as the frequency rises the response falls, and where it falls low enough the detail is gone. One curve, and it tells you what happens to every size of detail rather than to one.

🔬Why an MTF can rise above 100 per cent, and what that has to do with development

Look at a published MTF curve for a real film and you may find it climbing above 100 % at middle frequencies before it falls. A material cannot manufacture contrast out of nothing, so something else is happening, and it is not optical.

It is chemistry, and it belongs to the previous page. At an edge between a heavily and a lightly exposed region, the developer in the heavily exposed region is being used up and is producing by-products that inhibit development, while fresh developer diffuses in sideways from the lightly exposed region. The result, as the peer-reviewed treatment of the effect puts it, is that development runs more rapidly at the edge of the heavily exposed area, giving a local maximum of density there — the border effect — and more slowly just inside the lightly exposed area, giving a local minimum — the fringe effect. The eye reads that pair of overshoots as a crisper edge.

These are the adjacency effects, and they are why a dilute developer with gentle agitation can look sharper than a vigorous one: Kodak’s own D-76 sheet says a 1+1 dilution gives greater sharpness with a slight increase in graininess, and ILFORD’s data sheets carry a “maximum sharpness” column of developer recommendations distinct from their fine-grain one. The quantity that measures the result is acutance, and it is defined from the density profile across a knife-edge exposure.

Part VIII (acutance-adjacency-and-compensation) owns all of it. What belongs here is only the warning that sharpness is not a property of the film alone, so an MTF curve is always an MTF curve for a stated development.

Halation, the layers underneath, and how thick to make the emulsion

Photograph a bare lamp against a dark ceiling and it may print not as a point but as a disc with a diffuse edge. That is halation, and its cause was worked out by Abney in 1875.

Light that passes through the emulsion without being absorbed reaches the far surface of the base, where some of it is reflected back into the emulsion — most strongly, as theory predicts and as Abney confirmed by experiment, at the critical angle. It re-enters the emulsion some distance from where it came in, and exposes crystals there. The developed result is a ring of reduced silver around the image of a bright point, shaded inwards and outwards according to how much light was reflected at each angle.

The nineteenth-century cure was to back the plate: coat the rear surface with a material in optical contact with the glass, of nearly the same refractive index, that absorbs the photographically active light, and wash it off before development. Every modern film does the same job with a coated layer, and the manufacturers have not all chosen the same place to put it.

A modern film in section, on BERGGER's published layer scheme

Depths drawn readable, not to scale: the base is around a hundred times the emulsion. The layer names, the order and the base thicknesses are BERGGER's own for Pancro 400; other makers solve the same problem differently, as the note below sets out. Layer depths are drawn to be readable, not to scale: on real film the base is roughly a hundred times the emulsion, and drawn honestly the emulsion would vanish. Any thickness given in the labels is the real one.

📷Four answers to one optical problem, all from current data sheets

BERGGER undercoat the anti-halation layer, between the emulsion and the base on the same side, and say it clears during processing — their washing sheet notes that a sulfite rinse helps dissolve it.

ILFORD coat HP5 Plus with an anti-halation backing on the other side of the base, which clears during development; the base is 0.110 mm acetate in roll film and 0.180 mm polyester in sheet.

ILFORD SFX 200 dispenses with a layer and uses a grey acetate base, which the sheet says gives good halation protection — the base itself absorbs the stray light.

ILFORD Ortho Plus in 120 relies on the base’s own anti-halation properties, and in sheet form has a backing that clears in development.

Four engineering answers, one physical problem. It is worth noticing what they have in common: every one of them has to stop absorbing light before you look at the negative, so the anti-halation material must either wash out, dissolve or be a dye that bleaches. A layer that stayed put would be a permanent fog.

Emulsion thickness is the last of these material choices, and it is a genuine conflict. A thicker emulsion holds more crystals in the light path, so it absorbs more of the light and is faster and denser. But light scatters sideways as it goes through, so a thick layer spreads every edge; and development has to reach the bottom, so a thick layer develops unevenly and washes slowly. Thin coatings are sharper and faster to process; thick ones are faster to expose. The reason a modern film can be thin and still reach full density is covering power: filamentary silver, as the last page showed, gives its density with remarkably little metal, and Ware’s figure for modern silver-gelatin enlarging papers is a coating weight of only 1 to 1.6 grams of silver per square metre.

Looking for yourself

Grain is one of the few things in this course you can check with an instrument that costs less than a box of paper.

🧪What magnification you need, and how to find out before you buy

This course has not tested a specific USB microscope, and will not name a model or claim a magnification for one. What it can give you is the arithmetic to decide, which is more useful and outlasts the product.

The structure you are looking for is the mottle Kodak measure through a 48 µm aperture — call it a feature size of a few tens of micrometres. To see such a feature comfortably it should span something like ten sensor pixels. So the question to ask of any digital microscope is not its advertised magnification, which is meaningless without a screen size, but this: how many micrometres of the subject does one pixel cover? Divide the width of the field of view, in micrometres, by the pixel width of the sensor. If that comes out at 5 µm per pixel or better, a 48 µm feature spans about ten pixels and you will see granularity clearly. If it comes out at 50 µm per pixel, you will not.

Three things to look at, in increasing order of difficulty.

A developed print, in transmitted light through the paper’s edge or in raking reflected light. Print grain is the largest structure of the three and needs the least of your instrument.

A processed negative, backlit. Tape it emulsion-up over a diffuse light. Compare a dense highlight area with a thin shadow area and you will see that the mottle is strongest at middle densities, which is exactly why Kodak read granularity at a density of 1.00 rather than at the extremes.

Individual developed grains. At 0.2 µm these are below the resolution of any light microscope, because they are smaller than the wavelength of the light you would look at them with. No instrument you can buy will show them. That is not a limitation of your microscope; it is diffraction, and Part VI will make you calculate it. Sheppard and Mees could measure their grains in 1907 because the plates of 1907 had crystals ten to twenty times larger than a modern film’s.

Record what you see in the notebook with the material, the developer, the enlargement and the field width, because a grain observation without those four numbers cannot be compared with anything.

The choice a photographer actually makes

All of this comes down to one decision, taken before the exposure and impossible to reverse afterwards: which material, for this subject, at this print size?

The variables are not independent, and that is what makes it a judgement rather than a lookup. Speed buys you a shutter fast enough to stop movement, or a small enough aperture for depth of field, or an exposure short enough to hand-hold, and it costs grain and, usually, a little sharpness. Fine grain buys you enlargement, and it costs light. Print size decides how much of the cost you will actually see, because grain is only ever visible at a magnification.

✦Two worked judgements

A building, on a tripod, printed at 16 × 20 inches from large format. Take the slowest film you can expose. Nothing is moving, the camera is not moving, and the exposure can be as long as it needs to be — so speed buys you nothing at all, and the enlargement is small because the negative is big. Spend the whole budget on grain and sharpness. The one thing to keep an eye on is reciprocity failure, because a slow film at a small aperture in low light is exactly where the previous page’s correction tables start to matter.

A street scene, hand-held, in overcast light, printed at 10 × 8 from 35 mm. Now speed is the whole argument: without a shutter speed that stops the subject and your own hand, there is no photograph to print, and a grainy photograph beats a blurred one every time. Take the fast film. Notice also that the enlargement is only about eight times, so much of the grain penalty never reaches the print. The mistake here is not choosing the fast film — it is choosing a slow one out of a general preference for fine grain, and getting a sharp record of a moving blur.

The general rule underneath both. Work backwards from the print. Decide the size first, then the enlargement that implies, then how much grain that enlargement will reveal, and only then how much speed you can afford to buy with it. A photographer who chooses film by reputation rather than by print size is solving the problem in the wrong order.

And one honest note to end on. Grain is not only a cost. It is a visible texture with a long history in the medium’s own aesthetics, and photographers have chosen it deliberately for a century — pushing fast film to make it coarser, printing big to make it show. The engineering of this page tells you what you are paying and what you are buying. It does not tell you which you should want.

Recap

• A larger crystal is faster because speed follows projected area, and the effect was measurable by microscope in 1907: Bellach’s series ran 1.6, 2.3 and 3.6 µm from “very insensitive” to “very sensitive”.
• Size alone hits a ceiling, because a developed crystal is all-or-nothing, covering power falls as one over particle size, and a very large crystal wastes most of the photons it catches.
• The size distribution sets the curve. Narrow gives high contrast and a short scale; broad gives a long scale. BERGGER blend two emulsions of different grain size in Pancro 400 for exactly that reason.
• Habit separates speed from silver. A tabular grain has a large shadow and a small volume. Kodak’s own published figures put two ISO 400 films at granularity 10 (tabular) and 17 (conventional).
• Three things are called grain: the 0.2 µm microcrystal, the developed grain that replaces it, and the statistical clumping seen in a print. The third is tens of times coarser than the first, and a single crystal at a 12× enlargement is about forty times too small for the eye to see.
• Granularity is measured, graininess is perceived, and the bridge between them is magnification. Neither number means anything without the conditions.
• Resolving power depends on the target’s contrast: Kodak quote T-MAX 100 at 63 lines/mm at a contrast of 1.6:1 and 200 lines/mm at 1000:1. The modulation transfer function replaces both by reporting the contrast that survives at every spatial frequency.
• Halation is light reflected from the far side of the base, identified by Abney in 1875. Modern films fight it with an undercoat, a backing or a tinted base, and every solution must disappear during processing.
• The choice of material is made backwards from the print size, because grain is only visible at a magnification.

Check your understanding

Question 1. Two emulsions have exactly the same mean crystal size. One has a narrow size distribution, the other a wide one. Which is faster, which is contrastier, and why?

• The narrow one is faster and contrastier, because uniform crystals develop together
• The wide one is a little faster because its largest crystals cross threshold first, and the narrow one is contrastier because all its crystals cross within a small range of exposure
• They are identical in both, because the mean is the same
• The wide one is contrastier because it contains bigger crystals

Show the answer and why

Answer: The wide one is a little faster because its largest crystals cross threshold first, and the narrow one is contrastier because all its crystals cross within a small range of exposure

Speed is set by the first crystals to become developable, which are the largest, so a wide distribution containing some crystals above the mean gains a little speed. Contrast is set by how tightly the population is bunched: if every crystal crosses its threshold within a narrow band of exposure, density goes from nothing to everything over a short stretch of the log-exposure axis, which is a steep curve. That is why lith and copy materials are made as uniform as possible and why a general-purpose pictorial film is deliberately mixed, as BERGGER state on their own sheet for Pancro 400.

Question 2. Why can a tabular-grain ISO 400 film be finer-grained than a conventional ISO 320 one?

• Because tabular crystals contain less silver per unit area of film
• Because a flat plate presents a large projected area to the light while containing little silver, so it captures photons like a large crystal and contributes silver like a small one
• Because tabular crystals are more sensitive per photon absorbed
• Because tabular emulsions are coated more thinly

Show the answer and why

Answer: Because a flat plate presents a large projected area to the light while containing little silver, so it captures photons like a large crystal and contributes silver like a small one

Speed follows the shadow the crystal casts, because that is how much of the passing light it can intercept; grain follows the volume of silver it delivers when developed, because a developed crystal is all-or-nothing. A compact crystal ties the two together, since scaling a cube grows both. Flattening the crystal breaks the link. Kodak publish the result rather than merely claiming it: at ISO 400, T-MAX at diffuse rms granularity 10 against TRI-X at 17, both measured through a 48 micrometre aperture at a density of 1.00 in D-76.

Question 3. A 35 mm negative is enlarged 12 times and the print shows obvious grain. What are you looking at?

• Individual silver halide crystals
• Individual developed grains
• Statistical clumping and overlap of many developed grains, which produces structure tens of times coarser than any single crystal
• Dust on the negative

Show the answer and why

Answer: Statistical clumping and overlap of many developed grains, which produces structure tens of times coarser than any single crystal

Do the arithmetic and the first two options are excluded outright. A modern crystal is about 0.2 micrometres, so at 12 times enlargement it is 0.0024 mm on the print, and the eye resolves about 0.09 mm at normal viewing distance: forty times too small. The visible structure must therefore be much coarser than a crystal, and it arises from the random variation in how many developed grains fall in each small patch, together with the overlap of grains lying at different depths in the layer. Kodak size their granularity aperture, 48 micrometres, to that structure rather than to the crystal.

Question 4. A film sheet quotes a resolving power of 63 lines/mm and also 200 lines/mm for the same film. What is the difference between the two figures?

• One is with the film wet and one dry
• One is measured at a test-object contrast of 1.6:1 and the other at 1000:1, and a high-contrast target is far easier to resolve
• One is for 35 mm and one for sheet film
• One includes the lens and one does not

Show the answer and why

Answer: One is measured at a test-object contrast of 1.6:1 and the other at 1000:1, and a high-contrast target is far easier to resolve

Kodak print the test-object contrast beside each number precisely because a bare lines-per-millimetre figure is uninterpretable without it. Nearly black bars on white survive being smeared by scatter and grain far longer than two similar greys do, so the same film honestly resolves three times as much detail on the easier target. This is the central weakness of resolving power as a measure and the reason the modulation transfer function replaced it: an MTF curve reports how much contrast survives at every spatial frequency, instead of picking one threshold and one target.

Question 5. What is halation, and why must every remedy for it disappear during processing?

• Scatter within the emulsion; the remedy dissolves so the emulsion can swell
• Light passing through the emulsion, reflecting from the far surface of the base and re-entering the emulsion elsewhere; the absorbing layer must go because otherwise it would darken the finished negative permanently
• Fogging by the safelight; the remedy is a dye that bleaches in fixer
• Reflection from the camera interior; the remedy is a matt backing

Show the answer and why

Answer: Light passing through the emulsion, reflecting from the far surface of the base and re-entering the emulsion elsewhere; the absorbing layer must go because otherwise it would darken the finished negative permanently

Abney established the mechanism in 1875 and showed that a point of light develops as a ring, shaded according to the light reflected at each angle, with the strongest reflection at the critical angle. The nineteenth-century cure was to back the plate with an absorbing material in optical contact with the glass. Modern films use an undercoat between emulsion and base, a backing on the far side, or a tinted base, and all of them must clear during processing: an absorber that stayed in place would add density everywhere and behave exactly like fog. Note the last option describes camera flare, a real problem but a different one.

Question 6. You are choosing between a slow fine-grain film and a fast coarse-grain one for a hand-held street photograph in overcast light, to be printed at 10 by 8 inches from 35 mm. What settles it?

• The fine-grain film, because grain is always undesirable
• The fast film, because without a shutter speed that stops the subject and your hand there is no picture, and at about eight times enlargement much of the grain penalty is never seen
• The fine-grain film, because the enlargement is small so grain will not show anyway
• It cannot be decided without a light meter reading

Show the answer and why

Answer: The fast film, because without a shutter speed that stops the subject and your hand there is no picture, and at about eight times enlargement much of the grain penalty is never seen

Work backwards from the print, as the page argues. A 10 by 8 print from 35 mm is roughly an eight times enlargement, which reveals much less grain than the 12 times at which granularity is conventionally measured, so the cost of the fast film is small here. The benefit is decisive: hand-held work in dull light has a hard floor on shutter speed below which everything is blurred, and no amount of fine grain rescues a blurred negative. The third option gets the right premise and the wrong conclusion, since if grain will not show there is no reason to pay for its absence in shutter speed.

Check my answersClear