What is AF?

AF stands for Auto Focus, a feature that allows your camera to focus automatically while shooting.  In Canon EOS DSLR cameras, there are actually two completely separate focusing methods, depending on whether you are shooting images through the viewfinder, or using ‘Live View’ through the rear LCD screen for images as well as movies.

When shooting images through the viewfinder, an EOS DSLR will use a dedicated AF sensor, built in to the camera. As light enters the lens, a mirror bounces it on to the AF sensor, which will use a technology known as Phase Detection to accurately determine where the subject is. The camera then uses that information to drive the focus mechanism in the lens, to bring the image in to focus.

Shooting through the LCD screen

When shooting images or movies through the LCD screen in a DSLR, the reflex mirror is permanently moved out of the way, to send all the light straight on to the imaging sensor. This means that it’s not possible to reflect the light on to the dedicated AF sensor. 

For this reason, the camera needs to analyse the image as it hits the imaging sensor to determine how to focus the image. With this approach – also used by Mirrorless cameras such as Canon’s EOS M series – focusing technology is built in to the imaging sensor. Examples of on-chip focusing are Canon’s Hybrid CMOS AF, and Dual Pixel AF.