Of course, you would never walk around to recompose. My point is
that even if you have a focus lock, the camera is still
reevaluating the contrast at the focus point. If it sees a gradual
change in contrast anytime during the half-press, it will kick it
into AI Servo.
This becomes a problem if, while recomposing after a focus lock
(holding still until the beep and dot and everything), you pan the
focus point over a surface that is slanted away from you. The
camera perceives a gradual change in contrast (distance), and it
kicks into AI Servo.
To give an example, I was doing a fairly closeup shot of a person
and focused on the eyes, then I move the focus point elsewhere
while recomposing. If I pan gradually, say, across the face that is
slightly slanted away from the camera, AI Servo kicks in and will
not turn off. I think this is what causes the problem that everyone
is talking about. It doesn't happen every time, but it happens
enough to be a cause for concern.
BTW, I believe moving the focus ring while focus is locked causing
AI Servo to kick in is related to this. If you change focus, it's a
gradual change in contrast; and then you-know-what is triggered.
I think there should be a way to choose between AI Focus or One
Shot Focus in the menus for the creative modes. It's not really
adding a feature to the camera -- you still cannot choose to have
full time AI Servo (without a mono plug, that is) -- but it takes
the unpredictability away from those who want to do the old
fashioned focus and recompose. I really hope it's in a future
firmware update.
Anthony
1. Focus on whatever you want.
2. Wait for the focus conformation light in the view finder to come
on. (This should be prettry fast depending on what lens you are
using.) If you recompose before this, the camera is still trying
to achieve sharp focus and therefore would focus on the "wrong"
part of the image.
3. Recompose and shoot.
From here, the only way that I can have the AI servo kick in is to
play with the focus ring (can only do this with ring type USM
lens). And this is how I do it when shooting action.
Hope it works for you.
jlo
