Modern technology has made photography a lot more accessible. In the early days, photographers had to painstakingly focus their lens by hand before they took a photo. If they missed focus, the image (and the expensive film they were using) would go to waste. Now, almost all cameras, from your smartphone to a high-end DSLR, use autofocus to make it easier to get sharp shots.
Unfortunately, when the camera does everything in the background, many photographers don’t really understand what’s going on. They just point their camera, push the shutter button and hope that the camera will get the shot. If you want to really take control of your images, you need to know a bit more about autofocus and how to use it.
How Autofocus Works
Most modern cameras use passive autofocus instead of active autofocus. Rather than using a laser or infrared beam to measure the distance to the subject (active autofocus), passive autofocus uses a phase detection, contrast sensors, or often a mix of both. On a smartphone, the image sensor might double as the autofocus sensor. On a DSLR, there will normally be specific autofocus sensors embedded in the image sensor.
Although phase detection and contrast sensors use different methods, they both basically rely on areas with edges and contrast. The camera calculates what adjustments it needs to make to the focus of the lens so that the edges and areas of contrast are as sharp as possible. The logic is that when edges are sharp, they’re in focus. There’s a bit more going on when it comes to how the camera decides where the subject is in the frame, but we’ll address that in a moment.
These autofocus systems work great, in most cases. They fall down in low light, however, or when you are trying to focus on something that has no edges or contrast, like a flat blue sky or white wall. Your camera will normally still work, but in the worst cases, it will just take it a lot longer to find focus.
When you look through the viewfinder of a DSLR, you see a grid of dots or squares. These are autofocus points. Entry level cameras might only have a few autofocus points while professional cameras can have 60 or 80.
By default, most cameras will automatically select which autofocus point (or points) to use. The algorithms they use tend to assume that the subject of the image is somewhere near the center of the frame. It’s not a bad system, but it doesn’t give you a huge amount of control. If your subject is standing to the side, you might miss focus.
To get better photos, you need to take charge. With almost all cameras, you’re able to specify an autofocus point, or group of autofocus points, you want it to use. While there are too many variations to go into here, generally there will be a button, or combination of buttons, you press that swap between different autofocus point options. On smartphones or mirrorless cameras, you can often select an autofocus area just by tapping where you want the camera to focus on the touch screen.
Check your camera’s manual to find out more.
The Different Modes
As well as selecting an autofocus point, you can also select an autofocus mode. These tell your camera what to do when it’s looking for focus.
Single Autofocus Mode
One-shot AF (Canon) and AF-S (Nikon) modes are for static scenes like landscapes. Once your camera has found focus, it stays locked. If something moves in the scene—say, a bird flies through—it will be ignored. It’s the simplest to use and almost never misses focus.
AI Servo (Canon) and AF-C (Nikon) modes are for scenes with a lot of motion. Your camera will never stop adjusting focus. If you’re trying to track a football player as they run, it’s the mode to use. As the subject moves through the frame, the focus will constantly get adjusted. The issue with this is that if you’re trying to focus on a relatively still scene, your camera might jump the focus around.
AI Focus (Canon) and AF-A (Nikon) modes are a hybrid of single and continuous autofocus. When the scene remains static, the autofocus will lock. If something moves, it’ll adjust until it finds focus again. If you’re not sure what autofocus mode to use, it’s a safe and flexible bet.
Those are the basics of autofocus. More advanced cameras will have more advanced options buried in their settings. For example, the Canon 1D, 5D, and 7D lines let you configure exactly how continuous autofocus tracks subjects.
It’s worth taking the time to read your camera’s instruction manual and find out how to select autofocus points and modes; it will make getting accurate focus (and more importantly, sharp images) a lot easier.