The AF point array
How the focus point set-up differs between cameras
THE AF module in a DSLR sits below the
main mirror. Most of the light coming through the lens is reflected by the mirror up to the viewfinder, but some light passes through it and is reflected down to the autofocus module.
The AF sensors are part of this module, with the corresponding AF points visible on a screen in the viewfinder. Some cameras have the points etched onto a focusing screen, but ’transmissive’ LCD screens are now common in high-end bodies. These ‘intelligent’ viewfinders enable information to be overlaid on the image, including the active AF point(s) and the various ways that AF points can be arranged in groups or areas.