Phase detection autofocus: how your DSLR’s AF system actually works

Phase detection autofocus: how your DSLR's AF system actually works

Have you ever wondered how your DSLR’s AF system works? Unlike other types of digital cameras, DSLRs use a phase detection autofocus system to get your subjects pin-sharp. Knowing how this works helps you learn how to focus more efficiently, and our latest photography cheat sheet illustrates exactly what your camera is doing when you half-press the shutter button.

Focusing is one of those tasks that sounds like it ought to be easy, but often it’s not. In the old days, you simply turned the focus ring until the image looked sharp, or turned a ring on the lens to line up the index marker with a picture of some mountains, a group or a single person.

But today’s DSLRs offer much higher levels of resolution, and the old techniques of guesswork and estimates aren’t good enough any more. We want our subjects to be razor-sharp, and for that we need the precision of a modern multi-point autofocus system.

Understanding Phase Detection Autofocus

Phase detection autofocus uses the same basic principle as rangefinder cameras, superimposing two images ‘seen’ from slightly different positions – the distance between them indicates how far out of focus they are. In this case, the two images are taken from opposite sides of the lens and brought together on the AF sensor.

It’s a clever system, because the sensor can use the position and separation of these two images to work out how much to refocus the lens, and in which direction. This is why DSLRs still have the fastest and most responsive AF systems of all camera types.

How Phase Detection Autofocus Works: download our cheat sheet

Phase detection autofocus: how your DSLR's AF system actually works

Click on the infographic to see the larger version.

01 Main mirror
This is semi-silvered so that the image formed by the lens passes mostly up into the pentaprism and the viewfinder, but some passes through on to the 
sub-mirror behind.

02 Sub-mirror
This reflects the image down into the base of the camera where the AF sensor is located. The sub-mirror flattens against the back of the main mirror when it flips up during the exposure.

03 Autofocus sensor
Different DSLRs use different autofocus sensors – the pro-level D4 in our infographic, for instance, uses Nikon’s top-of-the-range 51-point Multi-CAM 3500FX sensor.

04 Light path: viewfinder
This is the route the image takes to reach the viewfinder…

05 Light path: AF sensor
… and this is the light path for the autofocus sensor.


10 reasons your photos aren’t sharp (and what you can do about it)
Best camera focus techniques: 10 surefire ways to get sharp photos
How to focus your camera for any subject or scene: free photography cheat sheet
Master your camera’s autofocus: which AF points to use and when to use them