The photographic ISO film speed rating has a long and sordid past which included several different names such ASA and DIN and conversions between them. ISO is an acronym for the International Standards Organization which establishes and monitors many standards in many industries, products, and services and the specific standard which applies to film speed is ISO 5800:1987. Today most countries and manufactures use this ISO film speed standard and so the confusion has been alleviated, that is until digital cameras became of age and started to use ISO ratings.

With a traditional camera the film speed or ISO is a measure of the film's sensitivity to light and higher ISO values like 800 and 1600 are more sensitive to light where ISO 100 or 200 are less sensitive. In a digital camera there is no film but there is an image sensor, usually a CCD, which has an adjustable gain amplifier and thus a sensitivity setting. Early digital cameras either had no ISO settings, a sensitivity setting or were equivalent to ISO 100 or ISO 50 only. Newer models are offering a compliment of ISO settings and as an example the Nikon Coolpix 995 has ISO100, ISO200, ISO400 and ISO800 settings.

Back to traditional films for a moment. A traditional film's increase in ISO represented an increase in the size of the silver bromide crystals used to cover the film. The larger these crystals are the faster the film, the finer the crystals the slower the film. The photographic result of larger crystals or faster film is that the image has less resolution and appears grainier. A digital camera has a similar trade off. As the image sensor's amplifier is set to higher gain this increases the dark current in the collector which in turn, based on the length of the exposure, adds noise to the resulting image. A cooled astronomical CCD camera has the same amplifier and noise potential but by cooling its collector the dark current is minimized.

The ISO standard has very detailed specifications and all are related to chemical film and not digital cameras. Most digital camera manufactures refer to their ISO setting as ISO equivalent and so we will treat it as equivalent. Table 1 below shows the various shutter speeds at various ISO equivalents and is based on standard exposure formulae. By moving horizontally in our table we encounter equivalent exposures. So a 1/60 of a second exposure in ISO 100 is equivalent to a 1/480 of a second in ISO 800. Similarly, a 7.5 second exposure @ ISO 800 would result, theoretically, in the same exposure for 60 seconds at ISO 100.