Automatic-Focusing.
There are two basic types of automatic focusing system; range-finding systems, and contrast or image-sharpness sensing systems.
- Range-finding systems send out a signal; usually an infrared beam (but sometimes ultrasonic), and see how long it takes for the reflected signal from the subject to come back. Windows can fool such systems, and they don't work underwater because the water absorbs or modifies the signal.
- Contrast sensing systems determine image sharpness in much the same way as the human eye, and are much more difficult to fool, but repeating patterns can still fool them, and they don't work if the subject lacks detail. Range-finding systems are sometimes called 'active autofocus' because they send out a signal, whereas contrast sensitive systems are called 'passive autofocus' because they don't. This is one case where it is usually better to be passive.
Additionally, autofocus systems can utilise the processing power of the cameras' computer to intelligently determine optimum focus
Multi-Point Focusing - The autofocus systems uses several different portions of the image to determine the proper focus.
Multi-Zone Focusing - Many digital cameras now offer multi zone focusing. The camera will automatically determine which zone (center, left, right, upper, lower) to use to perform the auto focusing. You no longer have to make sure that your subject is dead-center to be properly focused.
Hologram Laser AF - Sony's laser-assisted auto focus system on the Cyber-shot that uses a safe Class 1 laser to paint a grid on the subject that makes the auto focus fast and accurate.