AF Points | How many do you need?

Nearly all cameras have several AF points and will select the best focus point automatically - usually the one corresponding to the object nearest the camera. Most cameras allow you to choose the focus point yourself if you want to.

Below is a simulation of an 11-point focussing system in a digital SLR. The green rectangles indicate where the focussing points are, and those highlighted in red are the ones the camera is using to focus on this particular subject.

So how many AF points do you need? The number of AF points is used as a selling point for the camera, the assumption being that the more there are, the better the camera's autofocus system.

This is true, up to a point, but having lots of AF points can also be confusing. Sometimes you need to override the camera's selected focus point, and this can take longer when the camera's got 9, 11 or (on some Nikon models) 51 AF points to choose from. Don't forget that however AF points the camera has, it can only focus on one thing.

Cameras with many AF points may offer predictive autofocus, where the camera can follow the subject's movement across a number of AF points and anticipate the correct point of focus based on this movement. This may prove useful for action photography, though many photographers still prefer the simplicity of 'pre-focussing' manually.

Some older, simpler cameras may have as few as three AF points. This is not the disadvantage it might sound in general photography, since they'll be spread in a line across the frame and can cope with off-centre subjects very well. It's much easier to override simple systems like these too.

Indeed, many photographers prefer to use their cameras' centre AF point for focussing, even when the camera has many more.

Phase-detection AF | Fast but only suits SLRs

This is the type of autofocus system used by digital SLRs. Its advantage is that it's very fast. Its disadvantage is that it uses a separate sensor which needs to be in the light path from the camera lens. This isn't a problem in normal D-SLRs, but it's not compatible with live view modes, where the image is directed on to the main sensor instead.

Here's a simplified diagram which shows how phase-detection AF works. The sensor combines an image formed by the left side of the lens (colour-coded blue) and one from the right side (colour-coded red). From the overlap of the images and their relative position, the sensor can tell how far the subject is out of focus and whether its in a 'back focus' or a 'front focus' position. In other words, it knows which direction to move the focus, and how far.

Contrast AF systems use the image formed on the sensor itself. They offer more flexibility (more AF points, face-detection), but they're slower, which is why Sony has gone to the effort of including a fixed, translucent mirror in its a33 and a55 D-SLRs, so that the phase-detection AF sensor can be active all the time.

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.