Film Speed or ISO Rating

Different photographic emulsions on film vary greatly in their sensitivity to light—some require little, others need much light to properly record an image. This factor is known as film speed, which is indicated by a standard ISO rating. Low ISO values indicate "slow" films that need much light. High ISO values, in contrast, are typical of "fast" films that require little light. The latter have larger silver halide crystals in the emulsion, which tend to give a grainy structure to the image. Typical film speeds are displayed below. The main speeds are given in bold, and intermediate speeds are shown for one-third increments (Shaw, 1994).

12, 16, 20, 25, 32, 40, 50, 64, 80, 100, 125, 160, 200,

250, 320, 400, 500, 640, 800, 1000, 1280, 1600, 3200

The same concept is used for indicating the sensitivity of digital camera sensors. The standard setting of ISO 100 corresponds to the light sensitivity of an ISO 100 film and may be increased to 200, 400, or up to 3200 for high-end DSLRs. A higher ISO setting amplifies the signal from the sensor, so less light is needed for the photograph. However, this procedure also amplifies the sensor noise, both for intensity and color, which in turn creates the impression of a grainier image, quite similar to high-speed photographic film. Many aerial photographs presented in this book were taken with ISO 100 film speed or sensor setting, although some were taken at faster speeds up to ISO 800.

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