Now, of course there’s going to be that group of people that will insist you use manual mode always.
You can get some really sharp images using manual focus. However, that is outside the scope of this article since this is all about autofocus. The first thing we’ll look at is the difference between auto area focus and single point focus.
In an Auto-area autofocus, your camera decides what it should use as your focal point. It usually decides based on what looks most prominent in the viewfinder or closest to the camera. This might work when your subject is obvious and there are no potential distractions.
For more control, choose a Single-point autofocus setting. That mode allows you to choose your specific auto focus point (check your camera’s manual if you aren’t sure how to do this). After all, only you, not your camera, knows where you want to place your subject.
Most DSLR cameras give you four basic options for autofocus settings: single, continuous, auto or manual.
If your subject is not moving, choose “AF-S” for Nikon or “One Shot” for Canon. This mode locks in your focus based on the distance to your subject. This mode also allows you to recompose. Keep depressing your shutter button slightly, and focus remains sharp on your subject. Then you can move the camera slightly left or right, recomposing with your subject out of the center of the frame.
If your subject is moving, use continuous autofocus (AF-C for Nikon or AI Servo for Canon). In this mode, you place your autofocus point over your subject, and focus continues to adjust while you hold down the shutter button, keeping your subject in focus as it moves.
For example, if someone is riding a bicycle, place the AF point on your subject and slightly depress the shutter. As long as you are pressing the shutter, the autofocus will continually adjust to your subject, keeping them in focus as they move. When you are ready to take the photo, depress the shutter completely, and the camera will focus on your subject for a sharp image.
A third option merges the functionality of the single autofocus and continuous autofocus. This hybrid mode, (AF-A for Nikon or AI Focus for Canon), starts out as a single auto focus. Your camera won’t focus until you lock in on a stationary subject. Once you have your subject in focus, you can take the photo as you would in a traditional single auto focus mode.
If your subject starts moving, however, the autofocus releases and continues to track your moving subject. It gives you the best of both worlds.
One thing you should note is that almost every resource you’ll find on photography suggests that the third option of hybrid mode sounds a lot better than it actually works. I personally tried it and found that it didn’t work for me so I usually stick with either one shot or continuous focus.
Source: Digital Photography School