Pressing the shutter-release button halfway initiates autofocus. Focus mode lets you choose whether focus locks once the camera has focused or if the camera continues to adjust focus while the shutter-release button is pressed halfway.
AF-A (Auto-Select AF):
The camera automatically selects AF-S if the subject stationary and AF-C if it is moving.
AF-S (Single AF):
Use with stationary subjects. Focus locks when the shutter-release button is pressed halfway; the shutter can be released only if the camera is able to focus.
AF-C (Continuous AF):
Use with subjects that are in motion. The camera focuses continuously on the subject in the AF-area brackets while the shutter release button is pressed halfway (note that the AF-area brackets appear only in the detailed display). Photos can be taken whether or not the camera is in focus.
AF-F (Full-Time AF):
Use with subjects that are in motion. The camera focuses continuously; photos can be taken whether or not the camera is in focus.