Autofocus in modern cameras is unbelievable, but if you do not know how to use it correctly, it can be random and unpredictable. What you need to know about autofocus to get sharply focused photos with your DSLR or mirrorless camera.
How Autofocus Works
Autofocus is an integral part of modern cameras. They are simply not designed to be set manually.
There are somewhere between a dozen and about a hundred dedicated autofocus sensors or points on the image sensors of modern DSLRs (things are a bit more complicated and software-safe with mirrors.) Cameras, but the same principles apply). The autofocus points work in one of two ways: contrast detection and phase detection, although both rely on areas of edge contrast to find the focus. At Cambridge in Color, the process is well-distributed.
The autofocus points are not randomly positioned on the sensor. The center usually contains a core group that is mostly used, and then smaller groups at the edge of the frame when you need to concentrate on something that is not in the middle of the scene.
The three most important factors your camera focuses on are the brightness, contrast, and movement of the subject. Your camera can be more easily fixed on brightly lit subjects, especially if they are against a dark background or move. For this reason, the autofocus works so badly at night.
If you set your camera to autofocus wherever you want, it usually starts from the object with the highest contraction closest to the center of the image. If you want to focus in a different location, you need to take control.
Autofocus Points and Groups
In the default autofocus setting, your camera will most likely use all available autofocus points. Whatever your algorithms think is the most likely topic, it will select a focus point or a set of focus points to use. This is usually pretty good, but you do not have much control over the process and can concentrate on something in front of or behind your subject. In the photo below you can see that the camera has focused on the tree and the model's hand rather than on her face.
In addition to using each autofocus, 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 that you press to toggle the mode, and then a joystick or D-pad that lets you move your selection. If you are not sure, refer to the manual.
If a single focus point is selected, your camera will only try to focus on what is directly below that point. It does not matter what's going on in the rest of the picture, that's your topic.
Selecting a single autofocus point is the way to focus on a small subject. say, a bird or the eye of the model – in a busy scene. If your camera can focus at all, it will only be displayed directly under that one red dot in the viewfinder.
Groups of auto focus points or ranges share the difference between using a single autofocus point, which can be impractical leading to strange compositions – and the use of the entire sensor – that can be a crapshoot. Normally you choose between four and a dozen juxtaposed focus points, which then work as a group. Your camera will try to focus on the most likely subject below one of the selected points.
I usually use a group of auto focus points when I take pictures. It gives me the flexibility to tell the autofocus that it has to aim at the face of the model or at a group of trees, but I do not have to expect micromanagement of things. This is the best middle ground.
Single Continuous and Hybrid Autofocus
In addition to selecting an autofocus point, you can also control what your camera does when changing scene. There are three autofocus modes: Basic autofocus, continuous autofocus, and hybrid autofocus. We looked at them in detail before, so I'll just briefly recap here.
- Single Auto Focus Mode: Canon's One-Shot AF and Nikon AF-S focus this mode once and then remain locked. If the subject moves, the autofocus will not be adjusted automatically. This is for landscapes and the like.
- Continuous Auto Focus Mode: AI servo from Canon and AF-C from Nikon, this mode is the opposite of the simple auto focus mode. Your camera continuously adjusts the focus to where it thinks. It's great for sports or wildlife photography, but too jumpy for most things.
- Hybrid Autofocus: This mode, called AI Focus and Nikon AF-A by AION, combines the previous two modes. As long as not much changes in the scene, it acts like a single autofocus mode. When something moves dramatically, the focus shifts like a continuous autofocus. For static subjects this is a bit more choppy than the single autofocus, especially when there are background movements, but together with the selection of a small group of autofocus points it can be very reliable.
RELATED: What is autofocus and what makes the difference?
There is a button on the back of the camera that locks autofocus until you take a picture or press it again. For Canon cameras, this is the button marked with an asterisk (*). For Nikon cameras, this is called "AE-L". Locking the autofocus is useful when the subject in your desired composition is not directly under an autofocus point.
Select one of the options to use it Autofocus point and place it over your subject. Press the shutter button halfway to focus, then press the autofocus lock. Hold the shutter button halfway, recompose the picture to your liking, and take the picture with a perfectly focused subject.
Autofocus is getting better and more professional or professional Extended cameras give you more controls. They are far from being available on every camera, but the eye focus is the auto-focus of the eyes and the control of motion tracking in continuous autofocus.
With the autofocus of the eyes, your camera captures people's eyes. It's a flagship feature from Sony for the mirrorless line-up and is great for anyone shooting portraits.
With cameras designed for sports or wildlife shooters like the Canon 7DII, you can choose which subjects you're shooting and control how continuous AF reacts to their movement. Different motives move differently and require different types of tracking: A bird is just moving through the frame, while a tennis player is chirping from side to side. Having autofocus for your subjects will greatly improve accuracy.
Like all "automatic" cameras, autofocus is best when you are heavily involved in the process. If you choose your camera for yourself, you will not get the results you want.
Picture credits: Canon.