Get smart with autofocus

The essential settings and techniques to quickly take sharper pictures

EOS DSLRS have two autofocusing (AF) systems – the optical viewfinder, and the other for Live View. Both require some contrast in the image to work well.

When shooting with the optical viewfinder, the camera uses a dedicated AF sensor to work out when an image is in focus. The AF points you see in the viewfinder correspond with the focus detectors on the sensor. You can pick a single point or let the camera decide where to focus. If you activate all of the points (or a zone of them), the AF system will usually lock onto the nearest part of a scene, or area of highest contrast.

In Live View, the camera cannot use the AF sensor, so must use the imaging sensor to work out what’s sharp. Live View AF used to be ponderous, with the camera having to focus the lens back and forth to find the point of highest contrast. But Canon’s Dual Pixel CMOS AF tech, which sees each pixel on the sensor acting like an AF point, has made a big difference to Live View AF speed. But a classic optical viewfinder and AF sensor still has the edge.

It’s not just the number of AF points in the viewfinder that makes a difference, it’s their sensitivity too. Standard AF points have their sensors vertically or horizontally, and are effective at detecting lines of contrast that ‘break’ the lines of the sensor. ‘Cross-type’ sensors combine vertical and horizontal, while dual crosstype sensors also detect diagonal lines of contrast for greater precision.