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).