I get the following question rather lot – so when another reader asked a few days ago, I thought “let’s answer for everyone”.
When shooting, I usually use the center focus point (Canon 40D) to select what I want to focus on, press halfway and then recompose. I have recently gone through some pictures I have taken with Aperture 3, and have clicked “show focus points” and it shows that my focus point was off. It appears as though my camera did not lock the focus. I have done some research on different forums to find out what the problem could be, and some people discussed the modes “AI Servo, AI Focus, One Shot”… Should this affect the focus lock? Would I be better off changing which of the 9 focus points I wish to use for each shot rather than locking and recomposing? (I would rather not since it’s more time consuming!)
You are fine. You are using “one shot”, or you would not see a focus point displayed. You see, the “display focus point” function is only useful if you do not recompose, since the computer doesn’t know you recomposed. So the computer shows which point you used, but not where it was when you shot.
I.e. There’s no problems. The image is sharp where you wanted, right?
Your other question: yes, although the centre point is more sensitive, and is sensitive to both vertical and horizontal lines, it is usually more accurate to move the focus point. You can make this easier on many cameras by custom functions. But unless you are shooting with very narrow depth of field, you can usually get away with using the centre point and recomposing.