ISO sensitivity is the third factor besides shutter speed and aperture that determines the exposure of a picture is the light sensitivity of the electronic array that takes in the light and forms the picture. This is adjustable in most digital cameras and is usually called “ISO sensitivity.”

The term “ISO” is taken from an international standard measurement of film speed. While the measurement doesn’t directly apply to digital photography it has been borrowed to do so, and the sensitivity to light is displayed as ISO 200, 400, and so on. The higher the ISO sensitivity, the less light will be required to produce a given exposure. With greater light sensitivity, your camera can produce a given desired picture quality at a faster shutter speed and/or a narrower aperture. This is useful when you are taking pictures in dim lighting and don’t want either the blurred motion effect of a slow shutter speed or the narrow depth of field that results from a wide aperture. ISO was originally a measure of film speed, as in this roll of ISO 400 color film.

One might wonder in that case why light sensitivity wouldn’t simply be set always to the maximum, except when you want to produce one of those two effects (narrow depth of field or motion streaking). The answer is that with digital photography just as with analog photography, very high light sensitivity produces something called “noise.” This is the introduction of random marring that wasn’t in the picture as seen by the eye. The effect, whether using a very fast film or a very fast ISO sensitivity digital setting, is a grainy quality to the photo that is generally undesirable.

For that reason, there’s a trade-off between ISO sensitivity and the other factors that impact exposure (shutter speed and aperture) and it’s best to set the ISO sensitivity to as low a level as is practical given the prevailing light conditions. If the picture can be taken at a lower ISO sensitivity and still use a shutter speed fast enough and an aperture narrow enough to achieve the effect you want, it should be. At the same time, though, some photos do benefit from a higher ISO sensitivity. A photo that would normally require a flash can be taken without one using higher ISO sensitivity, avoiding the distorting effects of flash light. Also, a photo to be taken in dim light might be best taken with high ISO sensitivity rather than a slow shutter speed or a wide aperture. It all depends on what you’re looking for in the final picture.

All of these considerations are based around the idea of optimum exposure for the picture – the exposure that will render a photo that captures all elements in the scene well. It’s also possible to deliberately vary the exposure so as to either underexpose or overexpose the picture (in terms of the theoretical “optimum”) for a particular effect or emphasis. We’ll go into that more in a later section. For now, the important thing is to understand how aperture, shutter speed, and ISO sensitivity interact to set the exposure of the photo, and what the effects are of varying each of these three elements.

• Black, Brian. DSLR Photography for Beginners: Take 10 Times Better Pictures in 48 Hours or Less! Best Way to Learn Digital Photography, Master Your DSLR Camera & Improve Your Digital SLR Photography Skills