Autofocus modes (AF-A, AF-S, AF-C)

(http://digital-photography-school.com/avoid-blurry-photos-choosing-right-autofocus-mode/)

Are you using the Auto-area autofocus or Single-point autofocus selection?

Who gets to decide your focus point? That’s the question you are deciding with this option. 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.

Is your subject moving?

Most DSLR cameras give you four basic options for autofocus settings: single, continuous, auto or manual. To help you choose the right option, ask yourself, “Is my subject moving?”

No, my subject is not moving

If your subject is not moving, choose “AF-S” (single focus). This mode locks in your focus based on the distance to your subject. As long as your subject stays at that distance, your photo will be in focus. Your subject has to be stationary for this mode to work. In fact, your camera will not take the photo if your subject is moving (or it cannot lock focus).

This mode also allows you to recompose. Let’s say the autofocus point is in the center of the frame, but you want your subject on one side or the other. 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.

Yes, my subject is moving

If your subject is moving, use choose “AF-C”(continuous autofocus). 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.

No, my subject isn’t moving, but it might

A third option merges the functionality of the single autofocus and continuous autofocus. This hybrid mode, “AF-A”, 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 note of caution, I have noticed at times, if you recompose a stationary object quickly in AF-A mode, the camera can be fooled into thinking the subject is moving and release the autofocus.