Optimizing focus for the subject
Tuning the camera’s focus tracking systems to suit the way your subject moves is essential
For many years, it’s been possible to adjust the focus tracking capability of EOS cameras when they are set to AI Servo AF, though few photographers ever did. The EOS-1D X and the EOS 5D Mark III were the first models with AF ‘Cases’ – presets to tune the operation of the camera focusing system to specific subjects. All the AF Case does is change the settings for three AF tuning parameters at once.
The Canon EOS-1D X, 1D X Mark II, 5D Mark III, 5DS, 5DS R and 7D Mark II have AF Cases. The EOS 80D lacks AF Cases but has the same three AF tuning parameters in AF custom functions. The three tuning parameters are Tracking Sensitivity, Acceleration/ Deceleration Tracking and AF Point Auto-switching. Tracking Sensitivity is labelled as ‘AI Servo Tracking Sensitivity’ in the older EOS-1D models and the EOS 7D.
Tracking Sensitivity determines how the focus system responds when a new subject falls under the AF point. Positive values make the camera switch focus to the new subject closer to the camera, negative ones slow down the switch to a new subject. When photographing cars on a racetrack, if a fence post comes between you and the car, you don’t want the camera to refocus on the fence post, so a zero or negative setting is advised.
Acceleration/deceleration Tracking determines how the autofocus responds to the subject accelerating or decelerating. If the subject starts/ stops, then moves at speed, you need to increase the setting. For more consistent subjects – such as a runner on an athletics track – negative values help. Positive values can make the AF unstable if the subject moves only slightly for some frames in a sequence.
AF Point Auto-switching applies when you have more than one AF point active – either in AF Point Expansion or Zone AF modes. When the values are positive the camera will more readily switch to another AF point most likely to focus on the subject. If you’re using a wide-angle lens and the subject is small in the frame, the camera may focus using the wrong AF point.
Depending on your skill in keeping the AF point on the subject, you may need different settings than another photographer with the same camera, lens and subject.