CHOOSE THE RIGHT AF MODE

We all know what autofocus is, but how does it work? And what modes work best in different situations? Matthew Richards explains

All DSLRs have a conventional phase-detection autofocus module. The main reflex mirror is partially translucent, so light can pass through it and be directed by a second mirror, downwards to the autofocus module. This module contains a number of autofocus points, each with its own set of sensors. The sensors compare the light entering from opposite sides of the lens; if the light is in phase, the lens is properly focused; if the sensors measure a difference in phase, they can tell the lens in which direction focus needs to be adjusted, and by how much. The module keeps on checking while the lens is adjusting, until optimum focus is achieved. 

A basic DSLR typically has fairly few phase-detection autofocus points. In the case of Nikon cameras, for example (the principles here apply to all makes of camera, and to mirrorless models too), the D3500 has 11 AF points in total, of which only the central point is cross-type. A cross-type sensor enables greater accuracy, as it’s able to resolve detail in both horizontal and vertical planes, instead of just one or the other.

More advanced cameras tend to have more AF points. The Nikon D5600 has 39 points (9 cross-type), the D7500 and D750 have 51 points (15 cross-type) and the D500 and D850 have 153 points (99 cross-type). Greater numbers of AF points enable you to select positions at finer increments, over a larger area of the frame, as well as tracking moving subjects more accurately.