What is AF: how your autofocus system actually works

What is AF: how your autofocus system actually works

What is AF? We all know what autofocus is in principle, but how many of us really know how it works?

The autofocus system in your DSLR works by looking at the image, and then adjusting the lens using a motor. It can tell whether a shot is in focus by using the principle that a sharp image has a higher contrast than an out-of-focus one (for more on using autofocus, check out our guide to how to choose the best AF mode for your digital camera).

So rather than actively measuring the distance between camera and subject, it uses sensors that measure the image contrast at key points in the picture (a pair of sensors correspond to each AF point you see in the viewfinder).

By analysing the signal from each of the AF sensors, this system can adjust the lens to the point at which the highest-contrast image is achieved.

If more than one AF point is being used, the system assumes that you want to focus on the point that gives the closest focusing distance (find out how to use focus lock on your digital camera).

Still confused? Here’s a quick illustrated guide we made showing how your autofocus system works.

What is AF: how your autofocus system actually works

To show how your autofocus systems works we photographed this quick still life scene at home. Please don’t judge us by our record collection!

Focused Too Near

What is AF: low contrast

Here our lens was focused too near the area we wanted sharp – the O in the center of the Chopin record. As a result it gave a low-contrast image on the camera’s sensor.

Focused Too Far

What is AF: focused too far

Again, we’ve suffered from low contrast. Our lens was focused too far from the O in Chopin, giving us a low-contrast image on the sensor.

What is AF: In Focus

This time our lens focused precisely on the O, giving us a high-contrast image on the sensor.


DO or Di… Your lens markings explained
9 things you should know about using prime lenses
Manual Focus: what you need to know to get sharp images