Contrast-detection AF | Slow(ish) but simple and versatile

Contrast-detection autofocus uses the digital camera's sensor to check whether the subject is in focus. An out-of-focus image has lower contrast, an in-focus one has higher contrast, so the camera moves the focus in and out until the contrast is at its maximum and is therefore in focus. It's a simple, effective and versatile autofocus method, but not the fastest.

This diagram shows roughly how it works. When the image is out of focus, there's no way for the camera to tell, just from the blur, which way it has to move the lens or how far. This is why contrast-based AF systems tend to go through large focus movements even for shots where the subject distance hasn't changed. They can't be sure the subject is in focus at all without checking it at different lens positions.

Digital SLRs use separate phase-detection AF sensors which work on a different principle and are much faster. This is why digital SLRs have much slower autofocus in live view mode (where they have to switch to contrast-AF systems), and why Sony's gone to the trouble of designing a translucent mirror system for its a33 and a55 SLRs, which allows the cameras' phase-detection AF sensors to remain active all the time.

Having said that, some contrast-AF systems do seem much faster than others. Panasonic's G-series hybrid cameras seem as quick as regular digital SLRs, while Olympus's Pen-series hybrids are noticeably slower.