After re-reading the
Perfect focus everytime thread recently I remembered some lingering questions I had regarding AF.C (specifically with AF Select and Expanded Area AF settings). I did some online searches and now understand how autofocus works at a sensor level, and situations where it's useful (sports, kids, etc..). I still have some questions about using it in practice, though.
I know that focus depends on distance and depth of field, so obviously AF.C is great for keeping focus when the subject in the selected AF point moves in the Z coordinate... away from or closer to the lens. How does it work for lateral (X,Y) movement? In theory there's no change in focus distance if the camera moved in parallel to match the subject, but since there's usually angular change between subject and camera then there is probably some change in focus required as well (either more or less depending on how the subject is moving). Am I correct in assuming that AF.C will keep the focus as the subject moves left to right as well?
If so, does it only track the subject as long as it's kept on the selected AF point? If the subject moves to the right or left of that AF point, I'm assuming focus is lost.
When Expanded Area AF is enabled, does it keep focus only on the slected point and the surrounding (pink) points? Beyond that, focus is again lost?
Generally speaking, is the photographer supposed to maintain framing after focusing and keep the subject centered on the selected AF point at all times? Are you supposed to either pan the camera or zoom the lens to keep the framing while AF.C is active?
I appreciate your help on this. It's one of the last few technical aspects of my camera that I still don't understand. It might be helpful to explain using a scenario, such as a dancer walking off-stage (left-right movement) and a race car going around a bend (X,Y, and Z change) where maybe some zooming is also required to keep the composition as the car moves further away.