A photographer needs to lock focus on the subject in order for it to appear sharp in the image, and autofocus is a technology that accomplishes this task… automatically.
DSLRs use an autofocus sensor to analyze the light through the lens and figure out where the subject is located within the frame. You can help out the AF system by choosing a specific focus point and placing that point on the subject.
Different autofocus systems have varying degrees of accuracy. Some cameras are manufactured specifically for landscapes and portraits, and have mediocre autofocus systems.
These cameras might “hunt around” for focus in the dark and even fail to achieve focus in some extremely difficult lighting situations. Other cameras are engineered for sports and high speed shooting, and have autofocus systems that work very well even in difficult light. Also with mediocre AF systems, some AF points are less dependable on others. For example the center point may be very accurate but the outside points may have you hunting around for focus. This varies greatly between camera models and manufacturers.
An autofocus system needs a motor in order to adjust the physical elements of the lens. Sometimes this motor is built into the camera. Other times the motor is built into the lens.
All of Canon’s EF & EF-S lenses have a built-in focus motor. The cheaper lenses have less effective motors that focus slowly. Some high performance lenses have ultra-fast focus motors that lock on to the subject in a split second.
Some Nikon cameras have a built-in focus motor while others do not. Some Nikon lenses have a built-in focus motor and some do not. Due to this situation, Nikon photographers need to be aware of which lenses will autofocus on their cameras, and which lenses are safe to mount at all. Before choosing a lens for your Nikon you should consult the Nikon SLR Camera & Lens Compatibility Chart at Nikonians.org.
The various camera companies have different names for their high performance AF motors. Canon calls it USM for Ultrasonic Motor. Nikon calls it SWM for Silent Wave Motor. Sigma calls it HSM for Hyper Sonic Motor.
DSLRs generally have three autofocus modes:
A one-shot or single-shot mode for stationary subjects. The system locks on focus once and keeps it there while you finish making the image.
A continuous or servo mode for moving subjects. The system will track a moving subject and continuously adjust focus so the it stays sharp. You can see this in action by holding the shutter button halfway down (or holding the AF-ON button), aiming your lens at something that moves and following it around while continuing to hold the button. You’ll probably hear the AF system buzzing and if your lens has a distance window, be able to see it continuously adjusting the distance.
An auto mode that attempts to choose between single shot and continuous/servo mode each time you shoot. This is often the default mode for a camera.
Letting the camera pick its AF mode is fine in some situations, but if you can anticipate your subjects it’s good to make that choice yourself. This reduces the chances of missing focus and getting a bad frame because the camera made a poor decision. For example if you’re standing on the sidelines of a ball game, you should probably be in continuous/servo mode.
In dark situations where it’s hard for an autofocus system to find the subject and lock on focus, cameras will often use an AF assist lamp. This is a small light the shines on the subject before the shutter triggers, which the AF system can “see” and use to lock on.
On compact point & shoots this is the little red or green light that lights up right before you take a picture.
A lot of Nikon DSLRs have an AF assist lamp built into the camera body. Some Canon DSLRs have this function built into the pop-up flash, while bodies without a flash like the 5D have no AF assist light.
Many flashes have their own infrared focus assist beam, which is why obtaining focus in dark environments with a flash on top of the camera a lot easier than without a flash. I know of a lot of photographers, particularly Canon users, who like to use a flash solely for the IR focus assist light.
D.I.Y. AF Assist
Many photographers who do nighttime landscapes or just nighttime long exposures in general have a DIY trick for getting focus: set up the camera and tripod where you want them, then shine a flashlight where you would like to focus and trigger the AF. Turn off the flashlight and trigger the shutter.
I’ve mentioned the AF-On button a few times over the course of this article, so I’ll close by describing this very useful custom function. Many DSLRs have a custom function that allows you to remove autofocus from the shutter button and assign it to a separate button. This button is usually on the back of the camera near your thumb.
When this custom function is used, pressing or half-pressing the shutter button doesn’t trigger the AF system. The camera will simply meter the light and expose. If the subject isn’t in focus, it’ll turn out blurry. In order to get the subject in focus, you first press the AF-On button then hit the shutter.
This might sound complicated at first, but it’s one of those things that you’d never give up once you get used to it. I find it very intuitive to be able to focus and meter separately. It opens doors to many new techniques, such as dead simple focus & recompose, holding down the AF-on button to easily keep focus as you track a moving subject, and more. In fact the “D.I.Y AF Assist” I described in the previous section is much easier to do when this function is on because the AF system won’t be reactivated when you trip the shutter, so you don’t have to worry about holding a half-pressed button or anything.
Not all cameras have these types of custom functions, and the method for enabling them varies from camera to camera. Check your manual to find out how to do this with your DSLR.