Friday, June 27, 2008

Film Speed

Film speed describes a film's threshold sensitivity to light. The international standard for rating film speed is the ISO scale which combines both the ASA speed and the DIN speed in the format ASA/DIN. Using ISO convention film with an ASA speed of 400 would be labeled 400/27°. A fourth naming standard is GOST, developed by the Russian standards authority.

Common film speeds include ISO 25, 50, 64, 100, 160, 200, 400, 800, 1600, and 3200. Consumer print films are usually in the ISO 100 to ISO 800 range.

Some films, like Kodak's Technical Pan, are not ISO rated and therefore careful examination of the film's properties must be made by the photographer before exposure and development.

ISO 25 film is very "slow", as it requires much more exposure to produce a usable image than "fast" ISO 800 film. Films of ISO 800 and greater are thus better suited to low-light situations and action shots (where the short exposure time limits the total light received).

The benefit of slower films is that it usually has finer grain and better color rendition than fast film. Professional photography with static subjects such as portraits or landscapes usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure.

Photographing subjects such as rapidly moving sports or in low-light conditions, a professional will choose a faster film. Grain size refers to the size of the silver crystals in the emulsion. The smaller the crystals, the finer the detail in the photo and the slower the film.

A film with a particular ISO rating can be pushed to behave like a film with a higher ISO. In order to do this, the film must be developed for a longer amount of time or at a higher temperature than usual.

This procedure is usually only performed by photographers who do their own development or professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

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