|ILFORD Technical Service
Revised Film Speed Meter Settings
|The following readers query, and Ilford Ltd's reply (via a Technical Service Note) is taken from "Mailbag" in "35mm and Sub-Miniature Photography" magazine for December 1960.|
Sir,- As a regular reader of
your magazine, I would like to use some of your valuable space
in order to bring to light a rather confusing state of affairs.
I hope that this may lead to a clarification of the situation
by the company concerned, namely Ilford Ltd.
Could you explain the difference or, better still, publish a statement by Ilford Ltd?
Having discussed this matter with my friends, I find that a similar situation exists with regard to FP3 miniature film - perhaps we could 'kill two birds with one stone'.
BISHOP'S STORTFORD. GEOFFREY G. PERRY.
Sub-Miniature Photography" magazine replied:
We publish Ilford's statement below, Mr. Perry, and hope that it will help to clarify the situation.
A REVISION of American Standard
PH 2.5 - 1954 (Method of Determining Speed of Photographic Negative
Materials, Monochrome, Continuous-Tone) has recently been published
as ASA PH 2.5 - 1960. From the user's point of view the primary
difference between the revised and the original ASA methods is
the setting up of a new scale of speed numbers which, when used
with the majority of present-day exposure meters will result
in a camera exposure only slightly greater than the minimum necessary
for producing a negative of excellent printing quality. The purpose
is to reduce to a narrow margin the safety factor associated
with the Exposure Index defined in the previous American Standard.
It is expected that this second, logarithmic, system will take some time to become established, since camera and exposure meter dial markings will need to be altered to incorporate it. The new arithmetic system will, on the other hand, be taken into use immediately by photographers, many of whom have for some time made a practice of rating materials higher than the manufacturers' recommendations. Ilford Limited will therefore be altering immediately the meter settings on cartons, instruction leaflets and elsewhere to show the revised figures with minimum safety factor. The new American Standard does not specify a logarithm-to-base 10 speed similar either to that used by the old ASA and the current British Standard, or to that of the DIN, but the meter settings recommended for use with meters scaled in BS and DIN will also be altered immediately to minimise the safety factor. The revised figures will be identified by the heading "Meter Settings for minimum exposure". Only at some later date will the logarithm-to-base 2 system be featured.
At the present time only the American Standard has been published in revised form. It is expected however, that the appropriate British and German Standards will quickly follow on similar lines and that the way will then be clear to an International Standard. (Note: In fact, the relevant BS 1380 didn't finally arrive until 1963).
Ilford materials are listed below showing the old and new meter settings, including the log2 figures. It is stressed that the materials themselves remain unchanged.
Note: Although the above table refers to "35mm miniature film", this is believed to be simply because the query had been raised from 35mm Photography magazine. The author of this web page believes that the same advised speed changes applied to ALL black & white emulsions at that time, 35mm, roll film, cut film and plate.