Film speed

Speed is a measurement of how much light is needed to make a usable image on light-sensitive material such as film. "Faster" film requires less light, and so can be exposed at a faster speed or smaller aperture. Film speed is represented on a numeric scale; the higher the scale number, the less light that is required to expose the film. There are currently two numbering systems in use - ASA/ISO and DIN, but there have been various other predecessors.

"Fast" and "Slow" film

"Fast" film - typically with speeds of above 400 ISO/ASA - permit photography in low light, or the use of high shutter speeds and small apertures, allowing freezing of motion (and reduced camera shake), or large depths of field. In other words, fast films can result in sharper pictures.

"Slow" film - say, speeds below 100 ISO/ASA - can be used for slow shutter speeds - for deliberate blurring of motion, or large apertures to give selective focusing.

The speed of a film is specified for a particular, standard method of processing. Using non-standard processing - e.g. changing the development times, chemical dilution or using different developer chemicals gives the photographer a choice of a range of speeds for any given film. Increasing the speed of film this way is called "pushing".

Quality trade-offs

It is common for fast film to have larger grain size and poorer rendering of colours (or grey scale) than slow film. This means that slow film can generally produce higher quality results than fast, but this must be balanced against the motion stopping and depth of field available from fast films. Varying the development process - such as using a different developer - can also affect the quality and speed in similar ways.

Arithmetic v Logarithmic (Log) scales

There are two types of speed scales:

With an arithmetic scale, the exposure decreases in proportion to an increased speed - so a doubling in film speed requires half the exposure. For example if an exposure of 1/250s at f.8 is required for a 100 ISO film, a 200 ISO film would require either 1/500s at f.8, (or 1/250s at f.11) and 400 ISO film would need 1/1000s at f.8 for the same scene.

A logarithmic scale increases a fixed amount for a doubling of speed - 24 DIN is twice as fast as 21 DIN, and 27 DIN is four times as fast as 21.

Film Speed Scales

Conversion table on the back of a Gossen Sixon light meter showing old DIN, ASA/BS, Weston and Europ-Scheiner (log) scales

• ISO/ASA/BS - current arithmetic scale, defined in standards ISO 5800:1987 (international), ASA PH2.5-1960 (American), BS 1380 (1963, British)
• DIN - current logarithmic system - defined in the German standard DIN 4512 (1961); an increase of 3 is a doubling of speed. (21 DIN = 100 ISO/ASA and so 24 DIN = 200 ISO/ASA, 27 DIN = 400 ISO/ASA)
• DIN (old) - was written as a fraction over 10 (e.g. 21/10), and/or with a degree symbol, e.g. 21°. Old DIN was superceded by the 1961 DIN standard.
• ASA/BS (old)
• GOST (Cyrillic: ГОСТ) - a Russian arithmetic system. According to some sources, figures are about 25% larger than ASA/ISO (so 800ASA = ~1000GOST)^[2], but others quote the relationship the other way around, GOST is 10% smaller (800ASA = ~720GOST)^[1]. For most purposes, GOST ratings are near enough the same as ASA; this can be borne out by examining a Russian camera with a built in meter: many mark the same speed scale as GOST and ASA.
• Hurter & Driffield (H & D) - an obsolete British arithmetic system.
• Weston - an empirical arithmetic system, based on film developing tests, developed by the light-meter manufacturer. There was also a logarithmic (Weston-Scheiner) scale. Some other meter manufacturers had their own speed scales.
• Scheiner speed - one of the earliest logarithmic systems; there were several slightly different Scheiner scales.

Speed in Digital Cameras

Digital camera sensors also have a certain sensitivity to light, meaning that a particular amount of light (and so a particular exposure) is required to make an image - and this can be measured and specified in the same way as film. Many digital cameras have some adjustments available for this - often on a menu called ASA or ISO. Current (2008) cameras usually offer only a small range of adjustment, although high-end DSLRs are beginning to approach the range of speeds available for film.

Similar effects are seen in digital to film cameras; setting the speed high allows high shutter speeds, small apertures or lower light photography, but can produce more noise in the image (noise + pixel limitations are roughly a digital equivalent of grain). Slower settings allow more averaging, and hence lower noise and better image quality.

Sources

• [1] Focal Encyclopedia of Photography, Focal Press/
• [2] Leningrad 4 user manual