ISO film speeds

ISO (International Standards Organization) film speeds are based on the old DIN (Deutsche Industrie Normen) and ASA (American Standards Association) speeds. ASA speeds were in turn based on Kodak film speeds as established in about 1940. There is more about the original Kodak research in the paid module on exposing negative films and more about density and contrast in a free module on density, the H&D curve and gamma.

The basis of ISO speed for black and white negative films is simple. There are two criteria. The first is a fixed minimum density so that you get enough detail in the thin areas of the films, the 'shadows'. The second is a fixed contrast so that the negatives are neither too flat and muddy nor too contrasty.

For colour negatives, matters are somewhat complicated by the fact that there are three emulsions (red, green and blue sensitive): the speeds of all three must be taken into consideration. For colour slides (and digital sensors) matters are further complicated by the use of both maximum and minimum densities. It is easiest to begin with black and white films.

Jardin du Palais Royal, Paris

This dates from before the days when we used spot meters, and quite frankly, it is slightly under-exposed; there is no detail to be found in the negative in the deepest shadows.

Between the time she took this in the early 1980s and the time she started to use a spot meter in the early 1990s, Frances would often set the film speed as low as one half of the ISO speed (on a sunny day) in order to be sure of getting good shadow detail.

Here she used a Nikkormat and a 15/2.8 Sigma fish-eye, shooting on Ilford XP1. She printed the negative on Ilford Multigrade Warmtone.

fixed minimum density

The current ISO standard could hardly be simpler. Film speeds are based on the exposure required to give a log density of 0.10 above film base plus fog (fb+f). This requires only a bare minimum of explanation, but if you are not familiar with log densities you would do well to look at the free module on the subject.

Let's say that the film base has a density of 0.30. To visualize this, imagine a strip of film on a light table, with the emulsion scratched off a small area. Take a light reading from the table without the film, then take another reading through the film where the emulsion is scratched off. If the film base density is 0.30, the two readings will differ by one stop, which is a factor of two. The logarithm of 2 is 0.30, so this is your film base density. Densities are always measured logarithmically: 0.10 is a third of a stop, 0.15 is half a stop, 0.20 is two-thirds of a stop, and so forth.

The reason for scratching off the emulsion is that in a developed film, this has some density too. It should not be much, but it will not be non-existent. This is 'fog', the result of unexposed grains of silver being developed to a very small extent. If you take a reading through a part of the film where the emulsion is unexposed, but developed, you will get a slightly higher reading than through the film base alone -- hence the term 'film base plus fog'. A typical fog reading might be about 0.05, so by adding this to the film base density you start with 0.35. This would be 'fb+f'. If your fb+f is 0.35 and your overall density is 0.45, you therefore have a density of 0.10 above fb+f. In sensitometry it is usual to disregard the fb+f and measure only the image density.

The grey tint in 35mm film base is to help reduce halation (see the free Glossary) but mostly to reduce 'light piping' from the end of the film outside the cassette. Large format films and 120 roll-film do not normally have a base tint.

Rodchenko's toast

Roger shot this on Ilford HP4 Plus using a Linhof Technikardan 4x5 with 210/5.6 Rodenstock Apo-Sironar N. The rebate density (unexposed but processed film) is 0.06 and the darkest area in the beam of light passing through the glass has a density of 1.78 -- in other words, a density above fb+f of 1.72. The lighter side of the label on the bottle has a density of about 0.95 above fb+f; the darker side, about 1.35 ('about' because there are variations across the area). The lightest area of the negative with tone, behind the bottle, has a density of 0.03 above fb+f -- in other words, an absolute density of 0.09. A density of 0.03 is just about printable but it is disregarded in ISO speed calculations.

Scanned in colour -- a happy accident, we have to confess -- the area behind the bottle is left to go pitch dark, which was what Roger intended in the final shot.

As noted in the caption above, 0.10 is quite a high density: as little as 0.03 represents a printable variation in density. But 0.10 is a convenient number, easily measured with even a crude densitometer, and it corresponds quite closely to the point at which the contrast of most films starts to match the subject contrast reasonably reliably. At very low densities, below 0.05, quite large variations in subject brightness will often result in quite low variations in density. In the other direction, there is not much need to go beyond 0.10. But it is important to remember that the choice of 0.10 is essentially arbitrary: 0.08 or 0.12 would be just as good, and you could probably use 0.05 or 0.15 without any real loss of accuracy. To a modest extent, the choice of 0.10 explains the latitude for under-exposure of most black and white films: you still get a usable, printable density with a third of a stop or worse of under-exposure.

fixed contrast

The reason fixed contrast is important is obvious to anyone who has ever 'pushed' a film, increasing its development (and therefore its contrast) in order to get more speed. The ISO standard specifies a contrast equivalent to a gamma of about 0.62: the higher the gamma, the higher the contrast. For present purposes it does not really matter very much what gamma is but again you may want to check the free module which explains it more. There is a great deal more about all of this in our book Perfect Exposure.

Many photographers find a gamma of 0.62 or so perfectly satisfactory, but many don't. A lot depends on subject matter, lighting conditions, lens contrast, and enlarger type. Some prefer a higher contrast, typically up to about 0.70. Rather more prefer a lower gamma, 0.56 or lower. Understandably, those who prefer the higher gamma get more speed than those who prefer the lower gamma.

This sometimes leads those who prefer a lower gamma to say that films are never 'really' as fast as the manufacturers say they are. This is nonsense. The vast majority of films are very close to their ISO speeds under ISO conditions. Some are even above it, though rarely more than 1/3 stop. It is perfectly legitimate to develop to a lower contrast, and you should do so if that gives you the results you like, but as soon as you depart from ISO conditions, you cannot make judgements on ISO speeds. At this point you are dealing with exposure indices (EIs) rather than ISO speeds.


Ruined Spa, Slovenia

It was a cloudy day with intermittent weak sunshine, so there was no need to worry about excessive contrast. Frances set close to the full ISO speed of Ilford HP5 Plus (ISO 500 in Ilford DD-X) on her Pentax spot meter and based her exposure on the wall on the right, the darkest area in which she wanted texture and detail. There would have been little point in reading the interiors, as these were to read as black holes anyway. Alpa 12 S/WA, 35/5.6 Rodenstock Apo-Sironar-N.


film speed and developers

The choice of developer is important. A speed increasing developer can give two-thirds of a stop increase in true ISO speed, or possibly even a little more, so a film that is ISO 400 in a 'middle of the road' developer such as Kodak D-76 may well be ISO 650 or even faster in something like Ilford Microphen. In the opposite direction, there is virtually no limit to the speed that can be wiped off by a fine grain developer, but two thirds of a stop would be quite usual. In other words, the same ISO 400 film could fall to ISO 250 in something like Ilford Perceptol.

Most film manufacturers use middle-of-the-road developers for film speed testing, but ISO rules allow the use of any stated developer. The most egregious examples are Fomapan 200 and Paterson Acupan 200, where ISO 200 is reached only in speed-increasing developers: in D-76 or similar, ISO 125 is a lot more realistic, or maybe ISO 160 in most of the Paterson developer line-up except the extreme speed-increasing developers. They are excellent films, however, and the fact that so many people do use them at EI 200 and above is a testament to the fact that ISO speeds are not all they are cracked up to be. For that matter, Fuji Acros has turned out closer to ISO 80 in many developers, but there's less of a market for an ISO 80 film than for ISO 100.

There have also been honest mistakes. ISO standards require speeds to be tested for both 'green' ( recently coated) films and matured or aged films. When a new film is introduced, there is no two-year-old stock to test and the manufacturers have to guess at how the film will age. The most notorious example of this was the original Ilford Delta 400, which was ISO 250 (2/3 stop slow) or ISO 320 (1/3 stop slow) in most developers. The second version met its true ISO speed much more willingly.


Harmonium, St Clement's, Romney Marsh


Paradoxically, those who worry most about precise exposure are often those who need to worry least, namely, large format users. Even with severe over-exposure (a couple of stops) and lousy processing, 4x5 inch will wipe the floor with the finest 35mm.

Of course, many large format users rationalize this by saying that their exposure indices -- half the ISO speed or less -- are the 'real' speed of the film. They are talking nonsense, of course: the ISO speed is every bit as real as any personal EI, and a lot more consistent.

Roger used a Walker Titan SF for this shot, with a 210/5.6 Rodenstock Apo-Sironar-N and Ilford FP4.


ultra-fast films

Ultra-fast black and white films -- Fuji Neopan 1600, Ilford Delta 3200 and Kodak TMZ P3200 -- do not have ISO speeds corresponding to their box numbers. Neopan is slowest and finest grained and is ISO 650 or 800 at best; TMZ P3200 is 800 to 1000; and Delta 3200 is fastest (but coarsest grained) at 1000 to 1250. They are designed to give high speeds with the minimum possible increase in contrast, but many people prefer to use them at no more than 'push one' or even 'push half' over their ISO speeds: 1250 for Neopan, 1600 for TMZ, 2500 for Delta. On the other hand, when you really need the speed, TMZ can be pretty amazing at 12,500 and Delta can arguably go as far as 25,000.

Arles at night


Ilford Delta 3200 is our favourite ultra-fast film but we normally rate it at 3200 unless we really don't care about shadow detail. Frances has a 'benign essential tremor' (the medical term for 'shaky hands, but don't worry about it') and she can hand-hold fast wide-angles quite reliably: here a 28/1.9 Voigtlander on a Bessa-T.


colour film speeds

As already noted, speeds for colour negative films have to take account of the three emulsions. To a considerable extent, this means that they are limited to the speed of the slowest emulsion, or more accurately, to the speed of the slowest emulsion in the multi-layer coating after filtration, but as all three emulsions are closely matched, this is less of a problem than it might seem and the basic principles are the same as for monochrome negatives.

Slide films differ fundamentally. Speeds for negative films are based on density in the shadows. Within reason, it doesn't matter how much exposure the highlights receive: they can be ignored, or brought in with reduced development, softer paper, or selective shading ( burning and dodging -- a paid module).

If you used the same criterion for slides, the highlights would soon 'blow' to a featureless white (via increasing degrees of desaturation). The exposure must therefore be keyed to the highlights. If the shadows are too dark, they are simply left to go hang.

ISO speeds for slides are therefore based on two densities, the minimum and the maximum. Very occasionally, the ISO standard for slides leads to films that require more exposure than their ISO speed suggests. The original Fuji Velvia was the best known for this: many photographers found they got better results at EI 40 or even EI 32 rather than the nominal ISO 50. But, like negative ISO film speeds, slide ISO speeds are the best that we have, and better than gurus' private EIs.


D/log E curve and speed points for transparency films


Speed point 1 is 0.20 above fb+f and speed point 2 is either where the shoulder of the curve begins (where the rate of change of the slope shifts from positive to negative) or 2.0 above minimum density, whichever is lower. Here it is the former.

On this graph, the relative log exposure of 4 represents 1 lux-second

digital sensor speeds

These are set by analogy with ISO film speeds, and because they are analogies rather than determined in exactly the same way as film speeds, they are normally very accurate. It is however worth knowing that even slide films normally have more latitude than digital sensors, and negative films have a lot more latitude, especially for over-exposure. If you are going to err with digital exposures, it is usually best to err (very slightly) on the side of under-exposure. Using RAW files also helps.

modifying ISO speeds

It is perfectly OK, and indeed generally desirable, to establish your own personal exposure indices, but the best starting point is almost invariably the manufacturers' ISO speeds rather than the recommendations of some self-appointed guru.

If you don't have enough shadow detail in your negatives, cut the film speed in 1/3 stop decrements with each successive film until you do -- or if you have plenty of shadow detail, you may even care to increase the film speed in 1/3 stop increments. Do not be surprised if you rate an ISO 400 film as low as 200 or as high as 650, though if you go outside these limits, you may need to examine your exposure technique for negatives (paid module) unless you are striving for a particular result.

Likewise, if your slides are consistently too light or too dark, re-set the film speed: cut the speed if they are too dark, or increase it if they are too light. Remember that tired old shutters can give a great deal more exposure than new ones: we have one camera, some 40 years old, where the shutter consistently runs 1 stop slow, so an ISO 100 film can be 're-rated' at 200 and will give exactly the same densities as the same film rated at 100 in a newer camera.

Of course, if your slides aren't consistent, you will do well to re-examine your metering technique for slides, too...

Julie and Holly

Paterson Acupan 200 is quite possibly our favourite medium-speed film in 35mm, but it is only ISO 200 in speed-increasing developers; ISO 160 or even 125 may be more realistic in most developers. But under-exposing by 1/3 stop will still leave adequate shadow detail if metering is done properly, and in a shot like this, there isn't much significant shadow detail anyway, so 200 is sustainable. Roger used a Pentax SV and 85/1.9 Super-Takumar for this shot, developing the film in Paterson FX-39, where the true ISO is probably 160.

the bottom line

Even if they are imperfect, ISO speeds are a lot better than the Bad Old Days when speeds were set by the marketing department, not by scientific test. They are also a lot better than the maunderings of those who say "Of course, in the real world..."  In their particular real world, maybe; but each photographers effectively inhabits his or her own particular world, shooting different subjects with different cameras and different metering techniques.

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© 2005 Roger W. Hicks