Digital cameras rely on contrast to achieve focus. In other words they look for a point in a scene where there is a clear difference between light and dark and adjust the lens to make that difference appear as sharply as possible. However, there are two methods the camera can use. Many compact cameras use a simple contrast-detect system which scans the whole of the picture area looking for those differences between light and dark. More sophisticated models, and most DSLRs, use a phase-detect system, which involves splitting the signal to produce a pair of images and then comparing the two to see which is sharper and then adjusting the lens in whichever direction is needed to make the image sharper.
Phase-detect systems are very fast and accurate, but they use specific focus points to measure sharpness from. The more autofocus points the camera has the more accurate it will be. The points are usually linear, meaning the camera is looking for a change between light and dark tones along either a vertical or horizontal line.
Cross-type sensors look for changes on both the horizontal and vertical axes, making them even more accurate. Entry-level cameras may only have 7 or 9 autofocus points, with maybe only the centre point being a cross-type. More expensive models will have more sensors and more of them will be cross-type. For example the Canon EOS 7D has 19 autofocus points and they are all the more sensitive cross-type.
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Digital SLR Guide Author
This guide was written by Ian Younger
For the most accurate focusing it is an advantage to have a camera that allows AF point selection, which lets you choose a single point to focus on. With portraits, for example, it is important to make sure the eyes are sharp, so using a single focus point will make sure you can position it over an eye.
When shooting moving subjects it gets more difficult to ensure sharp focus. By the time the camera has achieved focus the subject has moved. It is essential, therefore, to look for a camera with a continuous focus mode, which will track the movement of your subject and try to keep it in focus as it moves. Canon call their single-focus mode 'one shot', while the likes of Nikon, Pentax and Sony call it AF-S.
The continuous focus mode is usually labelled AF-C, or, on Canon models, 'AI Servo'. Some models also have an option that will allow the camera to automatically switch from one mode to the other if it detects movement - could be handy if you are taking shots of your pet dog, for example, and it starts off sitting nicely but then suddenly gets up and starts to move around.
Another complication with focusing on moving subjects is that there is usually a slight time lag between pressing the button and the actual image being captured. The subject may be in focus when you press the button, but has moved out of focus by the time the actual image is recorded. Manufacturers like Nikon now use predictive focus technology, which effectively means the camera can measure the speed of a moving subject and take account of the shutter lag to predict exactly where it needs to focus when the image is recorded.