The Modern cameras from both Nikon and Canon come up with very advanced AF controls. One of those controls is the Release/Focus Priority control.
Both Nikon and Canon have it enabled for AF-C/AI-Servo.
This menu item lets you select what would get higher priority while shooting in Continuous focus mode, Acquiring Focus or Releasing the Shutter.
Obvious answer would be acquiring focus right? I mean why would you want to have an image which is out of focus? Let the camera ensure that it is in focus and then click.
Now this may seem to be the obvious answer but there is a tad bit more to consider. Lets take a look at both these options in a little more detail
- Focus Priority – Under this mode, the camera will trigger Auto-focusing and before shooting it will re-ensure that focus is locked on.
- Release Priority – Under this mode, the camera will trigger Auto-focusing and will not do a re-ensure procedure before clicking.
The problem with Focus priority is that it takes that extra time to re-ensure focusing. That extra step delays the camera from clicking and a lot of times for people who are clicking seriously fast action this can be an overkill.
If someone is seriously looking at the merits and demerits of the above two options, I am assuming that the person is good with his camera already and would have some idea about controlling DoF, If need be he/she would choose the Release mode for that extra speed and ensure that there is some element of scope by keeping a deeper DoF so that even if the focus is not slam bang on the eye of the subject, the DoF helps.
Please note that the camera does AF in both these modes. Its not as if there is no AF in the Release mode, its just that under the Focus priority mode it ensures that the focus is locked on before letting go of the shutter.
If you are not shooting fast moving action, then Focus Priority is the way to go forward. However if you are shooting action, I would tilt the scales towards Release Priority.
Following are the screens where you can set these for both Canon and Nikon