Autofocus

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Autofocus, or AF, is a system for focusing a lens using motors, based on information determined via the camera electronics.

Nikon's principal system for autofocus is phase detection, where images from two sides of the lens are electronically compared, telling the camera whether an object is in front focus or back focus. The cpu then instructs the autofocus motors, which refocus the lens. This continues until the desired area of the scene is in focus.

Cameras equipped with live view mode are also able to autofocus, in that mode only, using contrast measurement. This is based on the principle that contrast between two adjacent pixels is at its maximum when the image is in sharp focus. This method is slower than phase detection, and less reliable in lower light.

Phase detection autofocus benefits from lenses with wide apertures, such as f2.8, and good lighting conditions. Motors can be built into the camera or into the lens, as with AF-S type lenses. Some cameras, notably the Nikon D40 and Nikon D40X do not have motors in the camera at all, and therefore can only be used with AF-S lenses.

Phase detection is incompatible with linear polarisation filters, and with reflex lenses, also known as mirror lenses or catadiatropic lenses, because these change the phase of the light differently across different parts of the lens. A circular polarisation filter should be used when autofocus is required.