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.
As well as using every autofocus point, your camera will likely offer the ability to select individual points and groups of points or areas. There’s probably a button on the back you press to switch modes and then a joystick or D-pad you use to move your selection. If you’re in doubt, check the manual.
With a single autofocus point selected, your camera will only attempt to focus on whatever is directly under that point. It doesn’t matter what’s going on in the rest of the frame, that’s your subject.
Selecting a single autofocus point is the way to go when you want to nail the focus on a small subject—say, a bird or the model’s eye—in a busy scene. If your camera is at all capable of finding focus, it will only do it right under that one red dot in your viewfinder.
Groups of autofocus points or areas split the difference between using a single autofocus point—which can be awkward and lead to weird compositions—and using the whole sensor—which can be a crapshoot. You usually select between four and a dozen adjacent focus points that then work as a group. Your camera will try to focus on whatever is the most likely subject that falls under any of the selected points.
I usually use a group of autofocus points when I’m shooting. It gives me the flexibility to tell autofocus to target the model’s face or a group of trees but doesn’t require me to micromanage things. It’s the best middle ground.
Single, Continuous, and Hybrid Autofocus
As well as selecting an autofocus point, you also control what your camera does if the scene changes. There are three autofocus modes: single autofocus, continuous autofocus, and hybrid autofocus. We’ve looked at them in depth before, so I’ll just briefly recap here.
- Single autofocus mode: Called One-shot AF by Canon and AF-S by Nikon, this mode focuses once and then stays locked. If your subject moves, autofocus won’t adjust automatically. It’s for landscapes and the like.
- Continuous autofocus mode: Called AI Servo by Canon and AF-C by Nikon, this mode is the opposite of single autofocus mode. Your camera will continuously adjust focus to wherever it thinks it should be. It’s great for sports or wildlife photography but way too jumpy for most things.
- Hybrid autofocus: Called AI Focus by Canon and AF-A by Nikon, this mode combines the previous two modes. As long as nothing much changes in the scene it will act like single autofocus mode. If something moves dramatically, it will shift focus like continuous autofocus. It’s a little jumpier than single autofocus for static subjects especially if there’s background movement, but along with selecting a small group of autofocus points, it can be very reliable.
On the back of your camera, there’s a button that locks the autofocus until you take a picture or press it again. On Canon cameras, it’s the button marked with an asterisk (*). On Nikon cameras, it’s labeled “AE-L.” Locking the autofocus is useful when your subject doesn’t fall directly under an autofocus point in your desired composition.
To use it, select a single autofocus point and place it over your subject. Push the shutter button halfway to focus and then press the autofocus lock button. Now, keeping the shutter button half pressed, recompose the shot how you like it and take the picture with a perfectly in-focus subject.
Diving Deeper into Autofocus
Autofocus keeps getting better and better, and on more professional or advanced cameras, you get more controls. While they’re far from available on every camera, the two to watch out for are eye autofocus and control over motion tracking in continuous autofocus.
With eye autofocus, your camera will lock on to people’s eyes. It’s a flagship feature of Sony’s mirrorless line up, and it’s excellent for anyone shooting portraits.
Cameras designed for sports or wildlife shooters, like the Canon 7DII, let you select what sort of subjects you’re shooting and control how continuous auto-focus reacts to their movement. Different subjects move differently and require different sorts of tracking: a bird moves straight through the frame while a tennis player jinks from side-to-side. Having your autofocus set up for your subjects will significantly improve its accuracy.
Like anything “automatic” about cameras, autofocus is at its best when you’re heavily involved in the process. Just letting your camera do its thing won’t give you the results you want.
Image Credits: Canon.
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