When you take photo, you usually want part of it to be in focus. By making adjustments to the position of the glass elements in a lens, the plane of sharpness can be made to move either towards or away from the lens – this is focusing.
When you use your fingers to rotate the focus ring on the lens you are manually focusing. But modern cameras can do it all for you: they can automatically decide what you want in focus or can be guided by you (through selecting focus points and modes), and then move the lens to get as close as they can to the focus you want. That’s autofocus (AF).
Modern AF systems are fast and pretty accurate, but not infallible. Below, I’ll describe how the system works and why some of the problems come about.
Most modern cameras use one or both of 2 common types of AF systems – phase detect AF and contrast detect AF.
Contrast detect AF uses the image sensor (that actually captures the photo) to determine the best point of focus. As such, it can be used in almost any camera, and is the typical focus method for compact point-and-shoot and mirrorless cameras as well as DSLR cameras that autofocus in Live View. Contrast detect AF is generally slow compared to phase detect as it has to work by trial and error, but it’s very flexible in where on an image you can focus and usually pretty accurate for static objects.
In a DSLR, phase detect AF uses a dedicated autofocus sensor, usually in the base of the camera. It is fast and can be used with the mirror in the “down” position which means it can work while you can still see an image through the optical viewfinder of your camera.
Just to confuse the issue, many cameras now support on-sensor phase detect as well which use special pixels on the image sensor to speed up the focusing. However, for DSLR cameras these are only used when the camera is capturing in Live View mode[1].
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Sony SLT cameras are a special case that actually run in Live View mode all the time. They don’t have on-sensor phase detect pixels but can use the dedicated phase detect sensor all the time. ↑