Features

One of the major benefits of digital cameras over film cameras is the ability to adjust their sensitivity from shot to shot. Low sensitivity settings give the best-quality results, but require plenty of light. Higher sensitivity settings may reduce the picture quality, but allow you to photograph under lower light conditions and avoid camera shake.

While camera film is available in a wide variety of sensitivities, it's fixed for the entire roll. So if you picked a high sensitivity film optimised for low light, you'd be stuck with the same roll of film for, say, 24 or 36 exposures. You could still use it in bright daylight, but the quality would be compromised. With digital, though, you can adjust the sensitivity for every single shot if you like - it's incredibly convenient.

Photographic sensitivity is measured in units of ISO; this uses the same numbers as the older ASA scale. A measurement of ISO 100 or below is considered to be low sensitivity, or 'slow'. ISO 200 is generally considered to be normal sensitivity, and ISO 400 and above is high sensitivity, or 'fast'. Most compact digital cameras offer a sensitivity range of ISO 100 to 400, with newer models offering 800 or even 1,600 options. Digital SLRs typically offer ranges up to 1,600 or even 3,200 ISO.

Doubling the ISO figure doubles the camera's sensitivity, so ISO 200 requires half as much light as ISO 100; this in turn allows you to achieve the same result with an exposure that's twice as fast. A sensitivity setting of ISO 1,600 therefore requires 16 times less light than 100 ISO, which in turn allows you to achieve the same result with an exposure 16 times as fast.

Since higher sensitivities allow faster exposures and thereby reduce the risk of camera shake, you might imagine that they would be preferred for general use. As mentioned earlier, however, the picture quality deteriorates as you increase the sensitivity. The higher the ISO, the more electronic noise will appear on the picture. This looks like random coloured speckles, a little like 'snow' on badly tuned analogue TVs, and is particularly noticeable in darker areas. Most cameras employ noise reduction features, but use too much and you can smear out fine detail.

The amount of noise also varies between cameras. Compact cameras suffer from much more noise than a digital SLR at a given sensitivity. Both types of camera may produce clean-looking images at ISO 100, but increase the settings to ISO 400 and the compact's images will be much noisier. If you can increase both to ISO 1,600, the digital SLR's photos will still be acceptable, whereas the compact's will look horrible.

Most digital cameras are set up to adjust their sensitivity automatically, but for the best results you may want to set it manually. If the conditions are bright and you can hold the camera steady, select an ISO of 100 or 200. If it's getting dimmer, or you're experiencing camera shake with the lens zoomed-in, try increasing the ISO to 400 or 800. It's best to avoid ISO 1,600 or higher unless you really have no choice. The quality will be compromised, but it's still preferable to camera shake.

	1 High ISO sensitivities let you take indoor photos without a flash.

	2 High sensitivities also allow you to use quick exposures to freeze action.