The autofocus of modern cameras is incredible, but if you do not know how to use it properly, it may feel random and lustful. Here's what you need to know about auto focus to get crisp focused photos with your DSLR or mirror-free camera.
How Auto Focus Works
Autofocus is an important part of modern cameras. They are just not designed to focus manually.
There is somewhere between a dozen and one hundred or so dedicated autofocus sensors or dots on the modern DSLR image sensors (things are a bit more complicated and software-defined with mirror-free cameras, but the same principles are true). Autofocus points to one of two methods: contrast detection and phase detection, although both depend on areas of edge contrast to find focus. Cambridge in Color has a good division of the process.
The auto focus points are not randomly located on the sensor. There is usually a core group around the middle that will be used most of the time and then smaller groups towards the edge of the frame when you need to focus on something that is not right in the middle of the scene.
The three things that determine the most where autofocus will focus on the camera is the brightness, subject contrast, and subject movement of the subject. Your camera will find it easier to lock on bright subjects, especially if they are facing a dark background or moving. That's why autofocus appears so bad at night.
If you leave your camera for auto focus, wherever you want, it will generally lock on the highest contract field closest to the center of the image. If you want it to focus elsewhere, you must take control.
Auto Focus Points and Groups
In its default auto focus mode, your camera probably uses all autofocus points available to it and then based. Whatever its algorithms determine is the most likely topic, it selects a focus point or a series of focus points to use . It's usually pretty good, but you do not have much control over the process, and it can focus on something randomly in front of or behind your topic. You can look at the picture below as the camera has focused on the tree and the hand of the model instead of the face.
In addition to using any auto focus point, your camera will likely offer the option to select individual points and groups of points or areas. There is probably a button on the back you press to switch modes and then a joystick or D pad that you use to move your selection. If in doubt, check out the manual.
With a single auto focus point selected, your camera will only try to focus on anything directly below that point. It does not matter what happens in the rest of the frame, it's your subject.
Choosing an autofocus point is the way to go when you want to focus on a small subject – say a bird or model's eye in a busy scene. If your camera at all finds a focus, it will only make it right under the only red point in the viewfinder.
Groups of autofocus points or areas divide the difference between using a single autofocus point, which can be difficult and lead to strange compositions – and with the entire sensor that can be a sharp shot. You usually choose between four and a dozen adjacent focus points that then act as a group. Your camera will try to focus on what is the most likely topic that falls under any of the selected points.
I usually use a group of auto focus points when I shoot. It gives me the flexibility to tell autofocus to target the face of a mother or a group of trees but does not require me to micromanage things. It's the best middle class.
Simple, continuous and hybrid auto focus
In addition to selecting an autofocus point, you also check what the camera does if the scene changes. There are three auto focus modes: simple auto focus, continuous auto focus and hybrid auto focus.
- Single Auto Focus Mode: Called One-shot AF by Canon and AF-S by Nikon, this mode is focused once and then stays locked. If the subject moves, auto focus will not be automatically adjusted. It's for landscape and the like.
- Continuous Auto Focus Mode: Called AI Servo by Canon and AF-C by Nikon, this mode is the opposite of an autofocus mode. Your camera will continually adjust the focus to anywhere it thinks it should be.
- Hybrid Auto Focus: 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 as an autofocus mode. If something moves dramatically, it will shift focus as continuous autofocus. It's a bit jumper than an autofocus for static subjects, especially if there's background motion, but it's very reliable with a small group of auto focus points.
RELATED: What is autofocus and what is different from Modes Mean?
Auto Focus Lock
On the back of your camera there is a button that locks the auto focus until you take a picture or press it again. On Canon cameras, the button is marked with an asterisk (*). On Nikon cameras it is labeled "AE-L". Auto focus locking is useful when the subject does not fall directly below an autofocus point in the desired composition.
To use it, select a single autofocus point and place it over your subject. Press the shutter button halfway to focus, then press the auto focus lock button. Now you hold the shutter button halfway, the shutter compresses how you like it and takes the image with a perfect focus.
Diving Auto Focus Focus
Auto focus gets better and better and more professional or advanced cameras get more controls. While they are far from available on all cameras, the two that fit the eyes are auto focus and control motion tracking in continuous auto focus.
With auto focus in the eye, the camera locks on human eyes. It's a flagship feature in Sony's mirror-free line up and it's great for anyone shooting portraits.
Cameras designed for sports or wildlife players, like the Canon 7DII, let you choose the type of subjects you shoot and check how continuous autofocus responds to their motion. Different subjects move differently and require different types of tracking: A bird moves straight through the frame while a tennis player jinks side to side. If you have autofocus set for your topics, it will improve its accuracy.
As something "automatic" about cameras, autofocus is at its best when you are strongly involved in the process. Just letting your camera do its thing will not give you the results you want.
Image credits: Canon.