Digital Photography Technology

ISO stands for “International Standards Organization”. ISO film speed is a measure of its sensitivity to light. There is a specific correlation between light, lens opening, and shutter speed to properly expose film.

Since the beginning, when film was first used, there have been numerous methods in an attempt to rate ISO film speed. Some were proprietary and others were national. The most important one were: ASA ( American Standards Association ), Weston, BSI ( British Standards Association ), H&D ( Hurter & Driffield, DIN ( Deutsche Industrie Norm ), and GOST ( Gosudarstvenny Obshchesoyuzny Standart )

GOST is still used in Russia and some of the Eastern European countries. ASA and DIN have become the standards. ASA is a numerical representation of film speed and DIN is logarithmic.

An example of a film rating would be 50/15º and 100/18º. The first one would be ASA 50 and DIN 21. The second would be ASA 100 and DIN 18. The second rating would be twice that of the first. When ASA goes up twice, then DIN goes up by 3. This indicates a one stop change, either in the lens opening or shutter speed.

The ISO film speed ratings of digital and film are same. The difference between film and digital are that ISO rating of film is fixed at the time when the film if manufactured. In a digital camera, the ISO can be changed at any time.

A slow speed film is always a finer grain that a faster speed film, which means that a larger print can be made. Once the film is in a camera, you have to work with what ever film you have. In a digital camera, if light conditions change, the ISO can be adjusted to meet your requirements.

The change is sensitivity is made by an amplifier, as the sensor itself can not be changed. Amplifiers will introduce noise, so it is best to keep the ISO as low as possible.

In earlier camera the ISO film speed had to be set manually. Film makers started placing a bar code on the film which was recognized by the camera. This automatically set the ISO on the camera. The bar code also has other information about the film which could also be used by the labs when making the prints.

For the person with the fully automatic camera, there is no need to be concerned about ISO. For those that use cameras that have adjustments for lens and shutter, it is necessary to understand ISO, stops, and light in there relationship to the camera lens and shutter.