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ISO is an international standard method for quantifying film’s sensitivity to light, and the ISO settings on a digital camera represent an equivalent sensitivity of its sensor. While ISO on a digital sensor is different from that of film, a digital camera’s ISO equivalent settings do correspond to film, so that if you set a digital camera to ISO 400, you can expect a response to light that is similar to that of an ISO 400 film.

Low numbers, such as 50 or 100, represent a relatively low sensitivity. Higher numbers, such as 400

or above, represent higher sensitivity. ISO numbers are mathematically proportional to the sensitivity to light. As you double or halve the ISO number, you double or halve the sensitivity of the camera or film to light (i.e., 800 speed is twice as sensitive to light as 400 and is half as sensitive as 1600). This means that as less light is available, you can simply turn up the ISO setting so that the camera can get a good picture without having to use a shutter speed that is so slow that it compromises sharpness (see page 78 for an explanation of the relationship between shutter speed and sharpness).

Changing ISO picture-by-picture is easy with a digital camera. Being able to change ISO on the fly as needed is a huge advantage; for example, you could be indoors using an ISO setting of 800 so that you don’t need flash, and then follow your subject outside into the blazing sun and change to ISO 100 to deal with those conditions instantly.

You change ISO on most Canon D-SLRs by pressing the ISO button and, depending on the camera model, turning the Main dial or the Quick Control dial (up or down arrow keys on the back are also used on some models). On still other Canon digital cameras, ISO is set from the Function menu.

Technically, a digital camera’s ISO setting does not actually change the sensitivity of the sensor, which is why it is an ISO setting rather than a true ISO number like with film. Digital cameras adjust the “sensitivity” of the sensor circuits by amplifying the sensor signal that creates the image. This is, in a sense, like turning up the volume on your radio to better hear a station that doesn’t come in very well.

Higher ISO settings will increase the appearance of noise in your images. Though Canon cameras have a well-deserved reputation for minimizing noise, higher ISO settings always result in more noise compared to lower settings, regardless of the camera. But sometimes, when shooting action or in low-light conditions, for example, higher ISO settings are useful nonetheless.

It’s good to understand that different camera models have different responses with respect to the degree of noise affected by an ISO setting. Small-sensor cameras, especially the little pocket cameras, can be very sensitive to ISO change, and you will quickly notice noise as ISO settings increase from the camera’s lowest setting.

Canon D-SLRs offer images with little noise at low and moderate ISO settings. And even at relatively high ISO settings, they offer excellent results. This allows you to compensate for the use of slower lenses (lenses with smaller maximum lens openings) and for shooting in low light. In Canon D-SLRs with full frame sensors, you will often get images with minimal to no noise at any standard setting and very low noise at high settings.

The best thing to do, however, is to use the lowest ISO possible for the conditions, as this will give you the best color, tonality, and resolution possible. But, it is nice to know that you can increase the ISO settings when necessary, and when you do, you are getting image quality that is actually better than what was possible with film. For Canon D-SLRs, you can easily shoot at ISO 200 or 400 and get results very similar to what you would get with ISO 100 film.