af point precision

How the choice of lens you fit on the camera can affect the accuracy of the autofocus system

CAMERA manufacturers list aperture information alongside details of a DSLR’s AF points.

The reason for this is that the lens attached to the camera affects the precision of the AF sensors. A camera needs plenty of light to focus, and faster lenses – those with large maximum apertures – allow more light to reach the AF sensor module. It doesn’t matter if you dial in a different aperture to take a picture: it’s the maximum aperture that counts.

For example, the Canon EOS 5D Mark IV’s AF point array (above) features 61 AF points, all of which offer vertical sensors with lenses that have a maximum aperture of f/5.6. The majority of the sensors are also cross-type, with five sensors in the centre offering dual cross-type focusing with f/2.8 lenses.

In common with other high-end DSLRs, the 5D Mark IV also has a number of AF points that offer f/8 focusing, which is useful if you need to attach a teleconverter to the lens.