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.

As with exposure, digital cameras use different kinds of methods to collect focus information and then evaluate it to produce the correct focus. The image will be in sharp focus at the position of highest contrast. An autofocus system may rely on the ambient illumination on the subject, or use a special autofocus light source built into the camera to improve the lighting under dim conditions. of the pattern.

Autofocus Considerations

I outlined a few of them in the section on manual focus.

• Autofocus speed. The speed at which your autofocus mechanism operates can be critical. Because autofocus is built into the lens, rather than the camera, this speed is highly dependent on the design of the lens. Some models focus more slowly than others, either because focusing involves lens elements that can’t be shifted rapidly, or because the motors and actuators in the system move too darn slowly.
• Autofocus technology. Different digital SLRs use different autofocus systems, even among systems offered by the same vendor. These differences can involve the type and number of sensors used to calculate focus. Sensors can consist of lines of pixels that are evaluated, or cross-hatched sections that cover more area. Different sensors may be used in bright light than in dim light. Plus, you may be able to move the sensor’s viewpoint from place to place in the frame, using your camera’s cursor keys.
• Autofocus evaluation. How and when your camera applies the autofocus information it calculates can affect how well your camera responds to changing focusing situations. As with exposure metering systems, your camera may use the focus data from the various sensors differently, depending on other factors and settings. You use the equivalent of spot focus, center-weighted focus, and something that operates very much like averaging focus (although the camera will still select a point of interest to zero in on, no matter where it appears in the frame).