To save battery power, your dSLR doesn’t start to focus the lens until you partially depress the shutter release. But, autofocus isn’t some mindless beast out there snapping your pictures in and out of focus with no feedback from you after you press that button.

• Continuous autofocus. The chief drawbacks of this system are that it’s possible to take an out-of-focus picture if your subject is moving faster than the focus mechanism can follow, it uses more power, and it makes a distracting noise.
• Single autofocus. In this mode, once you begin to press the shutter release, focus is set once, and remains at that setting until the button is depressed, taking the picture, or you release the shutter button without taking a shot.
• Dynamic focus area. Because your dSLR has more than one focus sensor checking your frame, it may shift among them as focus is calculated. With dynamic area autofocus, the camera may automatically switch from using one sensor to a different sensor if it detects subject motion.
• User selected focus area. You switch from one focus area to another using the cursor pad.
• Nearest subject. In this mode, the autofocus system looks for the subject matter that’s closest to the camera, no matter where it is located in the frame, and focuses on that.
• Focus lock. Your digital SLR has a focus-locking button that lets you fix the focus at the current point until you take a picture.
• Focus override. Macro lock/lockout. Some cameras and lenses have a provision for locking the lens into macro position so focus can be achieved only within the narrower close-up range. Or, you might find a macro lockout feature, which keeps the autofocus mechanism from trying to focus closer than a given distance.
• Autofocus assist lamp. This is an optional feature that can improve autofocus operation in low-light situations.