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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.
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