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ISO Sensitivity ISO sensitivity is a property of the film or sensor in a camera. In film photography, ISO sensitivity is usually called film speed. When you buy film, the film speed will be clearly labeled on both the packaging and the individual film cartridges. In digital photography, ISO sensitivity is usually referred to simply as ISO. As the word sensitivity suggests, the higher the ISO, the more sensitive the film or sensor. If you are using a film camera, the ISO setting on the camera should in most cases match that of the film being used. This value generally remains unchanged until different film is loaded. If you are using a digital SLR, the ISO sensitivity can be varied at any time either by the photographer or by the camera. ISO (sensitivity) is one of the four factors of exposure. Not surprisingly, if the ISO sensitivity is increased while all other factors remain unchanged, the exposure increases. As with shutter speed and aperture, ISO affects more than just exposure. Another effect of ISO is grain.
Grain Grain refers to the inherently imperfect response of a film or sensor to the optical signal it receives. In film photography, the term grain arose from the fact that film uses microscopic clumps of chemical (called grains) that react individually to the light that strikes them. In digital photography, the sensor is composed of millions of rectangular elements which react to light individually. The imperfection in a digital sensor's response to an optical signal is frequently referred to as noise. Both noise and grain look very similar in a photograph, and they both become more pronounced as the ISO sensitivity increases.
ISO 400 (left); ISO 1600 (right)
Above, a very small region of two photographs has been blown up to show the grain associated with two different ISO speeds.
The ISO Scale Like aperture and shutter speed, ISO is generally selected from an accepted set of numbers referred to here as the ISO scale. While your camera may have other numbers or may not have some of these numbers, the scale given here is: 50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, 25600, 51200, and 102400. Note that for each stop of increase, the ISO value is twice that of the previous value.
Conclusion Don't be afraid to use a higher ISO just because of grain. In some cases, grain can add an artistic effect to photos. In general, however, it is best to only use higher ISO values when you know you need them. Such a case would be when you need a smaller aperture or a faster shutter speed than what you can achieve without increasing the ISO. It is probably not advisable to place too much emphasis on using a low ISO just to minimize grain. Doing so limits your usable apertures and shutter speeds. Only experience will give you a better idea what values you need, not just for ISO but for every user-selectable value in photography.
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