ABOVE: Vehicle AF being used with Face detection + tracking (left) and with Large Zone AF Horizontal (right). The active AF area displays differently, even though both methods are picking up the car.
Focusing operation
As well as choosing your focusing area, you also have a choice for ‘AF operation’ – One Shot or Servo AF. When shooting subjects that move, Servo AF is the more logical focusing mode to use alongside subject detection.
However, when using Servo AF the boxes will change position and appear to flicker. This is not the camera malfunctioning; this is simply illustrating that the system is working.
Single AF points
If you pick one of the AF methods that uses only a single focusing point – Spot AF, 1 point AF, Expand AF or Expand AF Around – then you cannot make use of the Subject detection technology. That’s because you’re in control of the position of the AF point and – as before – the only way the focusing point can move is for you to move it manually.
Size vs. quality
Some people have concerns that the boxes with Face detection + tracking are quite large, leading them to assume that the focusing will be less accurate than using other focusing set-ups. This is not the case, but again you need to trust in the programming.
If you are shooting portraits – whether human or animal – then turning on Eye detection AF will improve the accuracy of the focusing. However, if you want to get the whole person or animal in the frame, then focusing on a small area like the eye isn't as critical because you will be getting automatically a greater amount of depth-of-field. Why? Because you will most likely be working either with a shorter focal length or be further away from the subject.
Therefore pinpoint accuracy on the eye is no longer necessary: it is simply enough to focus on the main part of the subject.
