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ISO Rating (Film Speed)

ISO 100
ISO 400
ISO 800
ISO 100 (F4.0 1/4sec)
ISO 400 (F4.0 1/15sec)
ISO 800 (F4.0 1/30sec)

ISO Rating (also known as Film Speed or ASA) is, put simply, the sensitivity rating of the film. In digital cameras this term is used for the sensitivity setting of the image sensor.

In low light conditions we need a higher ISO rating in order to photograph without using electronic flash or a tripod. And similarly, in very bright conditions we will be able to use a lower ISO rating.

Generally, when we set a high ISO rating (In most film cameras, this is set automatically when the film is loaded), it produces a grainy image with higher contrast and sometimes lower colour saturation.

A lower ISO rating on the other hand will generally give high colour saturation and crisp, low grain images.

200 ISO film (or digital camera setting) is one F-stop (see Aperture) more sensitive to light than 100 ISO. Similarly, 400 ISO film (or digital camera setting) is one F-stop more sensitive than 200 ISO, or 2 F-stops more sensitive than 100 ISO.

This is important to know, as it can be used for effect, but is best avoided in some situations.

For example, Landscape photography commonly requires high colour saturation and nice sharp images. So its no good loading up on ISO 1600 film which would give grainy results.

It is also possible to force the camera to rate the ISO setting at something other than that marked on the box (see Film Processing).

Digital cameras use the same terminology as film cameras, but in reality it is only an approximation of the ISO setting of film. The light sensitive image sensor in the camera can be adjusted to be more or less sensitive to light using the same ISO rating setting.

Related Articles;

Film Processing


Shutter Speed

Film Vs Digital