I spent some time recently talking about manual focus with film SLR cameras, but I don't want to give the impression that I hate autofocus. In fact, I love it. Autofocus is fast, mostly reliable, and occasionally smart.
Messing with manual focus can be a cause of lost opportunities. Likewise, a shallow understanding of autofocus systems can cause missed or improperly executed shots. Newer autofocus systems (especially those on dSLR cameras) are getting smarter, but they still rely on some sort of input from the user. So here are some basics on the various autofocus options — read up and go experiment with them.
Do note that I'm covering the most basic settings and that some cameras may have more, less, or differently labeled options. Read your manual if my notes aren't making sense.
This most basic of the autofocus options. The “Auto” setting gives you a single autofocus when the shutter is depressed half way down. Take the shot and do it again — you'll get another autofocus. If you shoot in continuous drive mode (or rapid fire), the camera will attempt to refocus between shots. This will slow down your rapid fire rate, but each shot will be focused. This setting is good for most situations that don't require special setups.
The “Single” setting is very similar to “Auto”. The only difference is how it handles rapid fire situations. Rather than refocus between each shot, “Single” mode will focus prior to the first shot and keep that focus until you release the shutter button. The upside to this is that your rapid fire will run faster, but at the expense of possibly losing focus on moving objects. This setting is good for situations where speed is critical and your subjects are not moving across multiple focal planes between shots.
When it comes to moving objects, “Continuous” mode is the way to go. This setting is very different from “Auto” and “Single” modes because the focus is never really locked until the image is captured. Depressing the shutter half way will cause continuous focus tracking to activate. The camera will constantly adjust the focus as long as you hold that button down. This setting is good for sports photography or other situations where your subjects are moving in or out of focal planes.
Not really an autofocus setting, “Manual” is the opposite the other settings. This will require you to manually adjust the focus ring — depressing the shutter half way will do nothing for your focus. This setting is good for low light situations or any other time your autofocus is having a hard time getting it right. It's also the fastest way to get a shot off since the camera doesn't have to focus prior to capturing the image.