The autofocus in modern cameras is incredible but, if you don’t know how to use it properly, it can feel random and capricious. Here’s what you need to know about autofocus to get sharply focused photos with your DSLR or mirrorless camera.
How Autofocus Works
Autofocus is an essential part of modern cameras. They’re just not designed to be focused manually.
There are somewhere between a dozen and a hundred or so dedicated autofocus sensors or points on the imaging sensors of modern DSLRs (things are a bit more complicated and software-reliant with mirrorless cameras, but the same principles hold). The autofocus points work by one of two methods: contrast detection and phase detection, although both rely on areas of edge contrast to find focus. Cambridge in Colour has a good breakdown of the process.
The autofocus points are not positioned randomly on the sensor. There is usually a core group around the center that will be used most of the time, and then smaller groups towards the edge of the frame for when you need to focus on something that’s not right in the middle of the scene.
The three things that most determine where autofocus will focus your camera are subject brightness, subject contrast, and subject motion. Your camera will find it easier to lock onto brightly lit subjects especially if they’re against a dark background or moving. This is why autofocus performs so poorly at night.
If you leave your camera to autofocus wherever it wants, it will generally lock on to the highest contract subject closest to the center of the image. If you want it to focus somewhere different, then you’ll need to take control.
Autofocus Points and Groups
In its default autofocus mode, your camera most likely uses all the autofocus points available to it and then, based on whatever its algorithms decide is the most likely subject, it picks a focus point or series of focus points to use. This usually is pretty good, but you don’t have a lot of control over the process, and it can focus on something random in front of or behind your subject. You can see in the photo below that the camera has focused on the tree and the model’s hand instead of her face.