Autofocus point selection – the basics
Autofocus point selection is automatic when using the green square or auto modes on EOS cameras. The camera evaluates each autofocus (AF) point and the one(s) that are over the closest subject to the camera are prioritised. Blog reader Darrell commented on my post about zoo photography that he selects a single AF point and I realised there was a whole new blog post in reply 🙂 thanks Darrell.To be able to choose specific AF points from the many offered by EOS cameras it is necessary to use the creative zone modes; P (program), Tv (shutter priority), Av (aperture priority) and M (manual). It’s time to get off auto and make your own decisions as to where in the frame the subject is.
Choosing autofocus points
Choosing the AF point is simple – press the AF point selection button on the top right corner of the rear of the camera and then move the main dial, quick control dial, or even the multi-controller. Select a single AF point and place it directly on the intended subject. On many cameras the centre AF point is the most capable AF point and a lot of photographers just stick to that. It can make for boring compositions though as all the subjects tend to be placed in the centre of the frame as a result.
Going back to the zoo photography. Using a single AF point and placing it accurately on the animal will allow the camera to autofocus past the cage. This gives better pictures with the photographer in control of the camera.
Choosing the autofocus area selection mode
The EOS 7D was the first to include a function to change the AF point – not just the areas or grouping of AF points – but the actual active area that is used for AF. The same function is on the EOS 5D Mark III and EOS-1D X too. Normally AF points are sensitive over an area a little larger than the red boxes seen through the viewfinder, but selection of the Spot AF mode reduces the active area of each AF point so that it is no larger than the AF frame in the viewfinder.
Spot AF – good and bad
- AF point is smaller – you can more accurately place it on the subject and it will be less likely to pick up on nearby items
- The AF point is less able to track moving subjects, making it less than ideal for sport or action photography or if the photographer is not able to hold the camera and lens steadily on the subject.
- The camera will need the Spot AF area selection mode activated from the menu. It’s off by default.
Back button AF
Some photographers prefer to separate the AF activation from the shutter to either the exposure lock (*) button or the AF-ON button on the rear of the camera – this is referred to as back button AF. Back button AF allows the camera to be used primarily in AI Servo mode – ready for moving action. However with back button AF configured simply releasing the rear AF button will stop the autofocus, a bit like focus lock would do if one-shot AF was being used.