ISO - Meaning and Myth

ISO is a measure of the sensitivity of a film or sensor to light. But unlike film, which has a fixed sensitivity determined by the amount of silver present within the emulsion, the sensitivity of a digital camera is variable.

Setting the camera to ISO 400 effectively makes the sensor twice as sensitive to light as it would have been at ISO 200. By doubling the ISO you're effectively doubling the sensitivity of the sensor. As a consequence you only need half as much light to record an image. This feature enables you to shoot at a higher Shutter Speed than you otherwise would, enabling you to reduce the possibility of either camera or subject movement. One of the great advantages of a digital camera is the ability to change ISO, from frame to frame. When shooting film you need 2 or more cameras to have anything like the same flexibility. Otherwise you have to finish your film, prior to replacing it with a roll of different sensitivity/film speed.

Glenn Guy