Film speed rating

The ISO number indicates a film's speed

At one time, there were three film speed rating systems or scales in place. These were.

(1) ASA - the system of the USA Standards Institute, which used to be called the American Standards Association, hence the abbreviation “ASA.” (The term "ANSI" also briefly came into use to describe the same system. It was derived from the American National Standards Institute, another name by which the USA Standards Institute became known after the American Standards Association's organizational changes);
(2) DIN - a German standards system that expressed a film speed number followed by a degree symbol ( º ). DIN derives its name from the initials of the Deutsche Industrie Normen or Deutsche Industries Norm; and
(3) B.S. or BSI - British Standard, which employed the same numbering system as the American Standards Association (ASA).

The three systems used two different sets of numbers to rate the speed of films. For instance, an ASA 800 or B.S. 800 film speed was equivalent to a DIN 30 number. You will likely come across references to film speeds based on these different systems in older texts and manuals.

Note: EV or Exposure Value is not a film speed rating system, as some people seem to think. EV is a one-number substitute for a two-number shutter speed/aperture combination. See The Exposure Value (EV) system.

TODAY’S STANDARD SYSTEM - ISO

Nowadays, film speed is designated by a single, almost universally-accepted common system developed by the International Organization for Standardization - a worldwide federation of national standards bodies from some 130 countries - which uses the initials "ISO" before the film-speed numbers. ISO numbers show film speed in two ways, for example ISO 100/21. The numbers before the slash (e.g. 100/) are equivalent to ASA or B.S. numbers. The numbers after the slash (e.g. /21) are equivalent to DIN numbers. In essence, ISO film speed ratings combine both ASA (or B.S.) numbers and DIN numbers.

Just about any film you purchase today will have ISO numbering printed on its packaging to show its speed. However, you may come across a film package that contains only the numbers before the slash, (e.g. ISO 800 instead of ISO 800/30). This number may also be incorporated in the film's name, such as Fujifilm NHGII Professional 800 film. Designating a film ISO 100 is technically incorrect, since an ISO film speed rating number must incorporate both the ASA equivalent and the DIN equivalent numbers separated by a slash. For the sake of simplicity, you will find we have done the same thing in many places throughout PhotographyTips.com, often showing only the numbers before the slash when we talk about film speed.

In case you wish to look it up, the ISO organization's International Classification for Standards (ICS) for color negative film is "ISO 5800:1987 - Photography - Colour negative films for still photography - Determination of ISO speed." You can purchase a copy of the Standard on the ISO website.

WHY DO YOU SOMETIMES SEE EI 400/27 ON A FILM PACKAGE?

The letters EI are the initials for "Exposure Index," a measure of the film's light sensitivity under specific development conditions. Why use EI instead of ISO? Because the film requires a specific developer to be rated at ISO 400/27. If the film is developed in another developer, its film speed would be different, and therefore cannot be rated at ISO 400/27 under the ISO's system for determining film speed.

THERE IS A FOURTH SYSTEM

If you live in Russia or one of the former Soviet Bloc countries, you will perhaps be familiar with a fourth film speed rating code, known as GOST, the system developed by the All-Russian Scientific and Research Institute for Certification. Its structure is similar to the ISO system, where ISO 16 is equivalent to GOST 14, and ISO 2000 is equivalent to GOST 1800. Odds are, you will not encounter the GOST system unless you travel to Russia or a former Soviet Bloc nation. If you plan to (or if you already live there) and want to know ISO equivalents for the GOST system, you can refer to our sub-section under that heading. See Gost system equivalents.

WHAT DOES A FILM SPEED NUMBER MEAN?

The ISO numbers of film –– 100/21, 200/24, 400/27 –– indicate how quickly the film reacts to light. The higher the number, the faster the film will form an image. Film with an ISO 400/27 rating reacts twice as fast to the same light as ISO 200/24 film, four times as fast a film with an ISO 100/21 rating, and eight times as fast as film with an ISO 50/18 rating.

EQUIVALENCIES

BSI and ASA film speed codes use the same numbers for any given film. DIN numbers are different for the same films. The following provides equivalent numbers for the systems, and, taken together, shows the ISO range of film speed numbers - i.e. a film with an ASA of 50 and a DIN of 18 has an ISO of 50/18:

ASA 3/DIN 6
ASA 4/DIN 7
ASA 5/DIN 8
ASA 6/DIN 9
ASA 8/DIN 10
ASA 10/DIN 11
ASA 12/DIN 12
ASA 16/DIN 13
ASA 20/DIN 14
ASA 25/DIN 15
ASA 32/DIN 16
ASA 40/DIN 17
ASA 50/DIN 18
ASA 64/DIN 19
ASA 80/DIN 20
ASA 100/DIN 21
ASA 125/DIN 22
ASA 160/DIN 23
ASA 200/DIN 24
ASA 250/DIN 25
ASA 320/DIN 26
ASA 400/DIN 27
ASA 500/DIN 28
ASA 640-650/DIN 29
ASA 800/DIN 30
ASA 1000/DIN 31
ASA 1200/DIN 32
ASA 1600/DIN 33
ASA 3200/DIN 36
ASA 6400/DIN 39
ASA 12500/DIN 42

OLDER CAMERA?

You may have an older camera or light meter that has an ASA dial for setting film speed. Select the film's ISO number that appears before the slash (/) on your camera's or light meter's ASA dial.

TRIVIA ABOUT THE INITIALS “ISO”

You may wonder why the initials ISO are employed when the outfit that developed the system (the International Organization for Standardization) has IOS for its initials?

Bear in mind that the organization is multinational, so its initials are “IOS” in English but not in every language of the 130 countries it represents. The directors decided to use "ISO" internationally because “iso” is the root prefix of the Greek word “isos,” meaning "equal," a prefix that is found in a variety of terms, such as "isometric" (of equal measure or dimensions) and "isonomy" (equality of laws, or of people before the law).