Autofocus points made easy
Take control of the ‘eyes’ of your camera’s autofocus system
Your digital SLR’s AF mode determines how the camera focuses, while the AF motor built into the lens or camera
dictates the autofocus speed. But it’s the AF points visible in the viewfinder that control where the camera focuses – and how accurately it does so.
Autofocus is one of those camera features where you can see where your money is going. The more you spend on a body, the more autofocus points will be crammed into the viewfinder, and the wider the range of autofocus point selection modes and autofocus customisation options there will be.
If a camera has a viewfinder that’s densely packed with autofocus points, it has a better chance of accurately tracking a moving object as it moves through the frame, automatically ‘passing’ the subject from one autofocus point to the next. A large number of autofocus points can also make it easier to manually select one that lines up with the position of a small, off-centre subject without having to move the camera to focus.
Beginner DSLRs typically have around 9-11 AF points, while flagship professional models can boast anywhere from 60 to
It isn’t just a question of the number and density of autofocus points that you can see in the viewfinder: it’s their precision too
more than 150. But it isn’t just a question of the number and density of autofocus points: it’s their precision too. Standard AF points detect contrast in only one plane – vertical sensors, for instance, respond to lines of horizontal contrast that ‘break’ the line of the sensor – such as the horizon in a landscape photo. Cross-type sensors are more desirable, as they detect both vertical and horizontal lines of contrast, while dual cross-type sensors detect diagonal lines as well. You’ll need to use fairly fast lenses to activate this additional precision. (See ‘AF point precision’, above.)
It used to be the norm that a camera’s centre AF point was the only cross-type sensor in a sea of standard AF points, but the latest crop of DSLRs typically have a range of them spread throughout the display in the viewfinder – in fact, the entire AF point array can be made up of crosstype sensors in advanced bodies. This makes it easier to accurately focus on subjects that aren’t in the middle of the picture. In low light, these off-centre AF points can struggle to register contrast – you might notice the lens ‘hunting’ in and out of focus as it attempts to pick up the subject – in which case try the centre AF point. This is invariably the most sensitive of the lot, although you may have to compromise on the composition of the picture or use the focus lock and recompose technique. (See ‘Hitting the spot’, opposite.)
If you switch to Live View, you usually won’t see any AF points on the display. This is because the mirror is locked out of the way to feed the imaging sensor with the live image, so the AF module doesn’t come into play. In this case, autofocus is achieved by measuring contrast directly on the camera’s imaging sensor.
The exception to this is cameras that temporarily flip the mirror down to enable a picture to be taken using the normal autofocus system, before locking it out of the way again.