The Jump in Film Speeds

Article by Andrew Matheson in August 1960 PhotoGuide Magazine


THE JUMP IN FILM SPEEDS
Film speeds have a habit of changing when no one is looking. Here are the facts behind the recent ASA speed changes.
By ANDREW MATHESON, London

OUT of the blue, American film makers have doubled recommended emulsion speeds. What made them do it-and why didn't they do it before?
Speed is one of the most important film characteristics. Measuring it remained the biggest headache in the film industry for nearly half a century. Not because of a lack of methods, but through too many of them. It took nearly fifty years before someone hit on the idea of relating film speed to final print quality. One reason why it took so long was perhaps that it was a cumbersome way, outside and in the laboratory. But it brought order into a chaotic state of affairs. That was the beginning of the ASA (American Standard) exposure index numbers-and later the BS (British Standard) system.
For nearly thirteen years it worked like a charm. Photographers had a figure that they could set on exposure meters and use in exposure tables, and obtain correctly exposed negatives.
Last year the American Standards Association burst into this seemingly peaceful state of affairs. A new standard, made official a few months ago, in effect doubled all previous film speed ratings. At once we are at sixes and sevens again - or more precisely at sixes and dozens. What has happened?

MAXIMUM FILM SPEED
The ASA speed system worked like a charm - or nearly. In fact, some curious rumblings started up behind the scenes almost as soon as it appeared.
The first was the cult of maximum film speed. Some ten years ago clever photographers tried cooking a film in the developer for ten hours instead of ten minutes. They discovered that they got- in addition to a lot of fog- increased effective film speeds.

TEN TIMES INCREASE?
The actual speed gain was the source of many bitter arguments in clubs and photo journals, but seemed to range from a five to ten fold increase. Usually the eager darkroom beaver claimed that he took a film rated at 50 ASA, set his meter to 500 ASA, and exposed accordingly. After an overnight developing session, he proudly waved a strip of printable negatives. There was no cheating, but how did he do it?
Some of the speed gain was genuine - but not all tenfold of it. Finality development boosted the speed 2 to 2½ times (even if it ruined the negatives in every other respect).
Certain subjects could stand reduced exposures by having restricted tones, giving another doubling of effective speed. And finally, ASA film speed indexes (pre-1960) are designed to yield 2½ times over-exposed negatives as a safety factor against under-exposure. So 2½ x 2 x 2½ = 12½. Yes, maximum film speed claims were genuine - but misleading.

MINIMUM CORRECT EXPOSURE
It is this last factor, the latitude or safety factor against under-exposure, which has now got shot down. Actually, it had been under fire for some while.
From time immemorial the average photographer's bogeyman used to be under-exposure. As new concepts of image quality such as acutance came on the scene, together with new types of ultra-speed films, it became apparent that over-exposure was just as bad. With the fastest films, it seems quite senseless to over-expose pictures by a safety factor if you wanted to make the most of the enhanced film speed. And such films, more than any others, appreciably suffered in quality even on comparatively slight over-exposure.
Film manufacturers and users soon found that they got noticeably better results by using an appreciably higher film speed figure on their exposure meters. Films began to have two speeds: the official one and a second figure for "minimum correct exposure". Some manufacturers went so far as to quote two sets of figures for every emulsion on these lines. And plenty of photographers found that by using the higher of the figures they got as good - if not better - negatives.

DOUBLE FOR BLACK-AND-WHITE
To deal with this new confusion, the ASA speed system has been revised. Under the new system, emulsion speeds of black-and-white negative films become double what they were before. That simple process virtually eliminates the previous safety factor or margin against under-exposure. The confusion has not yet departed; for some manufacturers immediately doubled their official film speeds, others still keep quoting old ASA values. And it is a little difficult to make up your mind just where you are if you don't know who has done what, and whether a published ASA figure is an old speed index or a new one. In addition, the British Standard (BS) speeds are still unchanged for the moment, though probably they will eventually fall in line with the new ASA range.
The speed revision also cleared up a few outstanding points. One was to simplify the laboratory side of testing film speeds, and to bring it into line with another widely used system, the DIN speed criterion. When originally introduced nearly thirty years ago, the latter was itself a little unrealistic, for it measured speed under laboratory conditions of development which no amateur would ever use. The DIN standard was revised in 1958 to remove this difficulty. The new ASA testing method is very similar to the new DIN procedure; only the figures are different. On the other hand, DIN film speeds still are not minimum-correct-exposure speeds: the two-fold safety factor remains.

HOW IT AFFECTS YOUR METER
The somewhat complicated situation then boils down to this. To determine correct exposures, especially with exposure meters - whether separate, built into the camera, coupled with aperture speed controls, or completely automatic - we need a film speed setting. Most modern exposure meters are calibrated in ASA and in DIN figures.
With a film rated in ASA, we have to find out first whether it is the old or the new ASA figure. If it is the old one, set the exposure meter to double that speed. If it is the new one, use it as it stands. If the film carries a DIN speed rating, set the exposure meter to double the given figure.

NO JUMP IN COLOUR
All this applies exclusively to black-and-white negative materials. It does not apply to colour films, because no colour film rating system - with or without safety factors - yet exists. Colour film speed is not an entity measured in the laboratory; it is a manufacturer's recommendation. By rating a film at, say, 32 ASA, the maker merely tells us that if we set our exposure meter to 32 ASA for that colour film, we shall get correctly exposed pictures. Colour film has very little latitude anyway; exposures must be correct and neither over nor under. So for colour shots the recommended exposure meter settings remain what they were before.
One practical effect is that black-and-white films suddenly appear to be very much faster again than colour films. In reality, neither type of film has changed at all.

UNIT VALUES
ASA figures are known as "arithmetic". That means that the speed figure or exposure index is directly proportional to the actual speed. A film of 200 ASA is twice as fast, and needs half as much exposure, as a film of 100 ASA.

This page last modified: 19th April 2010