ILFORD Technical Service Note
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.
   

Speed Confusion

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.
I recently purchased a reload of HP3 miniature film,

On the carton,
the speed ratings were given as:
ASA, WESTON 200
BSI, SCHEINER 34°
DIN 24°

On the leaflet inside the carton,
the speed ratings were given as:
ASA, WESTON 400
BSI, SCHEINER 37°
DIN 27°


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.

   
"35mm and Sub-Miniature Photography" magazine replied:
We publish Ilford's statement below, Mr. Perry, and hope that it will help to clarify the situation.

REVISED METER SETTINGS: An Ilford Technical Service Note

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.
Two systems for expressing speed are laid down by the new Standard. The first (arithmetic) system results in a series of numbers which are approximately twice those resulting from the old ASA system when applied to the same photographic materials. The second system is based on a logarithm-to-base 2 scale and is intended to form an essential part of a simplified system for taking account of the variables in camera exposure, namely subject luminance, shutter speed, lens aperture and emulsion speed. This system will be identified as the Additive System of Photographic Exposure (APEX).

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.

35mm. MINIATURE FILMS

OLD METER SETTINGS
METER SETTINGS FOR MINIMUM EXPOSURE

ASA

BS

DIN

ASA

BS

DIN

Log2
HPS

400

37°

27°

800

40°

30°

HP3

200

34°

24°

400

37°

27°

FP3 Series 2

64

29°

19°

125

32°

22°

5.5°
PanF

25

25°

15°

50

28°

18°


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.

This page last updated: 28rd May 2012