Film speed numbers

Here's a question for all of you

I know the obvious: shutter speed is really the denominator of the fraction: 400 = 1/400 of a second. I know that f-stop is a logarithmic ratio representing focal length relative to size of aperture.

Does film speed (25, 50, 100, 400, etc) actually represent some physical property of film emulsion, or is it borrowed from shutter speeds because it conceptually corresponds so well?

Aperture is not logarithmic. It is simply the focal length divided by the opening of the aperture. That is why it is often written as f/x.

ISO speed is not based on a physical property of the film, but an amount of light needed to achieve a certain density. ISO 5800 is based on shadows.

Film speeds represent a character of the film.

It is measured in a certain procedure which is standardized for the very standards ISO, ASA, DIN, Scheiner, Gost etc.

However this procedure was not necessarily representive of the photographers practise…

The more recent standards start from a certain density above the fog level at a given development. The first DIN standard for instance gave only the gamma to be processed to (0.8).
The ASA standard started also from a level above fog but instead of precribing a certain gamm it characterized a certain point with a given position with respect to that point above fog.

Anyway, that resemblance of ISO 400 and 1/400 sec is pure accidental. (By the way 1/50sec and 1/400 are no langer standard shutter speeds). However, both the common speed scale and the ISO scale are arithmetical.

As the mathematical deduction of the arithmetical ASA speed figures is not that easy to understand I refer to the algorithmic DIN system:

The film is esposed under standardized circumstances under a wedge with marked steps of log 0.3 which is equivalent of a factor 2 and makes one aperture step too.
The steps are numbered at tenfolds of 0.3 as 3,6,9,12 etc. That very step that produces that minimal density above fog gives by its number the speed figure in DIN values.
For instance 21 DIN. In former times it was written as 21/10° DIN, as the DIN values were named as degrees DIN an that denominator was linked to that tenfold I hinted at above.

It may be true that manufacturers, when faced with the ability to modify a film speed over a range, choose speeds that approximate shutter speeds.

I would be surprised to see a film ISO of, for example, 179. I think that that film would be a hard sell in the market.

Matt

MattKing said:

I would be surprised to see a film ISO of, for example, 179. I think that that film would be a hard sell in the market.

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It wouldn't comply with ISO 6 guidelines for marking film speeds either. Though you could calculate intermediate speeds using the method and formulae in the ISO, the ISO gives the well-known third-stop series as the speed designation to be used.

Best,
Helen

Helen B said:

It wouldn't comply with ISO 6 guidelines for marking film speeds either. Though you could calculate intermediate speeds using the method and formulae in the ISO, the ISO gives the well-known third-stop series as the speed designation to be used.

Best,
Helen

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Helen:

Thanks for this - I didn't even know they had rules about this issue .

I'm curious though - when the guidelines themselves were developed, I wonder if the officially accepted ISO speeds were chosen because they were the speeds which people would be comfortable with (125, rather than 150, as an example).

Matt

Perhaps the close approximation of ISO speeds to shutter speeds is intended as a way of following the Sunny/16 "rule"? Maybe, we're so often used to built-in light meters nowadays that we sometimes forget when a lot of folks just used the "rule" and then "adjusted" the f-stop based on how cloudy it was?

Film speed is a characteristic of the emulsion. It represents the minimum amount of light needed to produce a certain density on the film. Film speed is determined by a strict and rather complex ISO standard (actually a set of standards for black and white, color, and some special kinds of film). The standard defines what the standard speeds are, how the speed is related to exposure, and the exact method of exposure, development, and measurement used to determine the speed. The standard ISO speeds consist of a pair of numbers, like 400/27 (followed by the degree sign). The first number corresponds to the old American National Standatds Institute (ANSI) speed. The ANSI speed is a linear progression with 1/3 stop intervals (100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, etc.) where a doubling of the number indicates a doubling of the film speed (an emulsion twice as sensitive to light). The second number corresponds to the equivalent German DIN speed. That sequence is logarithmic. These days you usually just see the first number. The reason for the two numbers was to accommodate light meters for the two older standards. Most modern meters use the ANSI numbers.

If you use the ISO speed with your camera or light meter, you will usually get reasonably well exposed pictures. But people do not expose and develop film the way the ISO standard does. Serious photographers often take the ISO speed only as a starting point, and they perform exposure and devlopment tests to determine a personal exposure index (EI) for the film that reflects how they use the it.

nworth said:

Film speed is a characteristic of the emulsion. It represents the minimum amount of light needed to produce a certain density on the film. Film speed is determined by a strict and rather complex ISO standard (actually a set of standards for black and white, color, and some special kinds of film). The standard defines what the standard speeds are, how the speed is related to exposure, and the exact method of exposure, development, and measurement used to determine the speed. The standard ISO speeds consist of a pair of numbers, like 400/27 (followed by the degree sign). The first number corresponds to the old American National Standatds Institute (ANSI) speed. The ANSI speed is a linear progression with 1/3 stop intervals (100, 125, 160, 200, 250, 320, 400, 500, 640, 800, 1000, etc.) where a doubling of the number indicates a doubling of the film speed (an emulsion twice as sensitive to light). The second number corresponds to the equivalent German DIN speed. That sequence is logarithmic. These days you usually just see the first number. The reason for the two numbers was to accommodate light meters for the two older standards. Most modern meters use the ANSI numbers.

If you use the ISO speed with your camera or light meter, you will usually get reasonably well exposed pictures. But people do not expose and develop film the way the ISO standard does. Serious photographers often take the ISO speed only as a starting point, and they perform exposure and devlopment tests to determine a personal exposure index (EI) for the film that reflects how they use the it.

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Yep, sounds like the Sunny/16 rule to me too!

nworth said:

The first number corresponds to the old American National Standatds Institute (ANSI) speed. The ANSI speed is... Most modern meters use the ANSI numbers.

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That US-American speed standard originated when that standardisation organisation was still named `American Standards Association (ASA)´. Thus `ASA´ was used, and still is used with reference to that obsolete pair of speed designation.

AgX said:

The first DIN standard for instance gave only the gamma to be processed to (0.8).
The ASA standard started also from a level above fog but instead of precribing a certain gamm it characterized a certain point with a given position with respect to that point above fog.

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That is not my understanding. In fact, it's pretty much the exact opposite of my understanding.

The original DIN standard was, I believe, for processing to gamma infinity (the maximum gamma obtainable before fog started to bring the contrast down again) with a fixed density speed point.

The original ASA standard was based on the Kodak fractional gradient criterion, which greatly reduced the importance of development time. The speed point was taken as a point where the tangent of the slope of the D/log E curve was 0.3x that of the tangent of the slope between the speed point and another point 1.5 log units to the right.

This was theoretically superb but hard to determine so the later (post-1959) ASA standard and the current ISO standard are based on a fixed density with a gamma criterion that looks a bit odd but approximates surprisingly well to the fractional gradient point. This is 0.8 log density at a point 1.3 log units to the right on the exposure axis.

For those who want lots of theory on this, read Perfect Exposure, which I co-wrote with my wife Frances Schultz (David & Charles/Amphoto 1999). The text was checked by a member of the ISO film speed committee so it's reasonably accurate -- though all errors are ours, not his.

Finally, the ISO arothmetic standard for 1/3 stop speeds seems to me to be based on common sense. You're going to want round numbers such as 100, and the smallest meaningful variations are 1/3 stop (which is also the interval of the DIN log standard), so it all flows from there.

Yes, you could have ISO 73 or ISO 140, and indeed ISO testing gives you those numbers on occasion; but by convention (and ISO standard) these are rounded to the nearest 1/3 stop number, usually upwards, though with monochrome, they are sometimes rounded downwards to give a fraction of a stop more latitude for under-exposure. For example, I'd expect more FP4 Plus (ISO 125) to be ISO 130-140 than ISO 110-120. Do not forget batch variations, which are much smaller today than they used to be but which were commonly +/- 1/3 stop as recently as the 1970s.

Several film speeds don't correspond to traditional shutter speeds: you get ISO 12, 25, 64 and 80 but seldom 1/12, 1/25 (except on old shutters), 1/64 and 1/80.

Cheers,

Roger