ISO Speed Standard for Film Speed

ISO Speed Film Speed
Camera that adjusts ISO Speed

ISO 6:1993 Photography — Black-and-white pictorial still camera negative film/process systems — Determination of ISO speed specifies the numerical scale that measures photographic film’s sensitivity to light, also known as film speed. Film speed is important because relatively insensitive film (referred to as slow film) requires more exposure to light to produce the same image density as a more sensitive film. The higher the sensitivity, the grainier the image will be. Because of these distinctions, engineers and scientists have long been interested in properly standardizing measurements of film speed to enhance image quality. Two of these standards forms have officially fallen out of use, but they exist almost entirely in the current standard of ISO speed.

There was significant interest in sensitometry, the science of determining the sensitivity of photographic materials, and its measurement in the early Twentieth Century. In 1930s Europe, DIN, Deutsches Institut für Normung (German Institute for Standardization), utilized the information that German researchers had accumulated in the past to create the DIN system, which used a base 10 logarithm multiplied by 10, and expressed the value in degrees. According to this logarithmic measurement scale, a 3-degree (3°) increase doubles the film’s light sensitivity.

Around this time in the United States, Weston Electrical Equipment Association and General Electric introduced film-rating systems to incorporate into their products. Motivated by this, ASA used research from Kodak to establish a new approach for film speed in ASA Z38.2.1-1943, which, after a few revisions, became ASA PH2.5-1954, a standard establishing the criteria for ASA speed. This film speed scale was arithmetic, and an image featuring a speed of 200 ASA was twice the speed of one with 100 ASA.

 DIN and ASA speed

In 1960, after the method to determine film speed was refined, ASA speed underwent a significant revision that doubled the film speed of many black and white negative films. In this same year, ASA published ASA PH2.5-1960, which established an additional logarithmic scale to complement the arithmetic scale. According to this revision of the standard, 100 ASA was equal to 5° ASA.

ASA stands for the American Standards Association, the organization that, of course, was later renamed as the American National Standards Institute (ANSI).

The ASA and DIN scales were merged together to create the ISO scale, which was first introduced in the original 1974 version of ISO 6. ISO speed makes use of the arithmetic scale from ASA and the logarithmic scale from DIN, but the latter expression is generally omitted. For example, film speed at a value of 50 ASA and 18° DIN has an ISO speed of 50/18°, but it would likely be expressed as just ISO 50. Even though these units are designed for actual film, they still apply to the sensors that respond to light in digital cameras. Digital cameras have the advantage over film of being able to adjust the ISO speed with each shot, while traditional cameras require the set ISO speed on the camera to match that of the roll of film being used. ISO speed ratings for digital still cameras (DSC) are intended to harmonize with film ISO speed but are not exactly equivalent due to differences between electronic and film-based imaging systems.

In addition to ISO 6:1993, there are several other standards for ISO speed. These include:

ISO 5800:1987 – Photography — Colour negative films for still photography — Determination of ISO speed and ISO 5800/COR1:2001 Corrigendum 1
ISO 2240:2003 – Photography – Colour reversal camera films – Determination of ISO speed
ISO 12232:2019 – Photography – Digital still cameras – Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index

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