A DSLR camera has (usually) three different modes of focusing which are set with a control. These are AI servo focusing, one-shot focusing, and manual focusing. One-shot focusing allows you to use one control in one position (usually depressing the shutter button partway) to automatically focus on the focal point (by default the center of the frame, but this can be reset to some other location), after which the picture is taken and the focus is lost.

You have to do the same process again to focus on a different focal point (or on the same one). This is the way autofocus used to work on all cameras prior to the digital revolution, and it’s still the most common, popular, and easy to use method. AI servo focusing allows you to hold down the shutter control and take pictures in rapid succession, click, click, click. The camera focuses automatically on the focal point (again, by default and normally the center of the frame) before each picture is taken. This allows you to take multiple shots of a moving target far more easily than you would be able to do with one-shot focusing. Finally, manual focusing is just that. You turn the camera’s AI off and use the Force – that is to say, you set the focus manually by looking through the view port and seeing whether it looks like it’s in focus or not. This is a good idea when doing macro photography, because the tolerances for focus are quite small, and the autofocus is more likely to make small errors that translate into a blurred subject when taking close-ups. It’s also a good technique for shooting landscape pictures at night, when the autofocus finds it difficult to lock onto the target. Most DSLR newer cameras allow you to combine autofocus and manual, letting you fine-tune the focus after the autofocus has locked in. As long as you have the time to use it, this is often the best of both worlds.