06-10-2010, 11:57 PM
I used to have the non-L 100 until I was given the L one for my birthday (lucky me). The old one focussed just fine. There's not that much difference between the old one and the new one; the new one in my tests gives a slightly better and contrastier result wide open and has IS. It's preferable, but the expense is quite high and I don't think I would have upgraded on my own.
If your careful tests reveal that it focusses fine, there is no point getting it calibrated either.
So I'm guessing that it may be, as others have indicated, error due to focus and recompose.
There are 2 sources of error with this:
the first is that you are probably focussing on the eye, which is is likely in the upper part of the frame.
If you point the centre AF point at the eye, and then recompose with it pointed lower, the distance between sensor plane and the eye has changed slightly. Enough to be noticed in some applications. You could try using the uppermost single AF point. Even though it's not cross-type it might work better.
The other source of error in thin DOF photography is slight swaying forward and backward by the photographer or the subject after focus lock and before exposure. The more recomposition, the longer the time between them so the more chance of sway.
If your careful tests reveal that it focusses fine, there is no point getting it calibrated either.
So I'm guessing that it may be, as others have indicated, error due to focus and recompose.
There are 2 sources of error with this:
the first is that you are probably focussing on the eye, which is is likely in the upper part of the frame.
If you point the centre AF point at the eye, and then recompose with it pointed lower, the distance between sensor plane and the eye has changed slightly. Enough to be noticed in some applications. You could try using the uppermost single AF point. Even though it's not cross-type it might work better.
The other source of error in thin DOF photography is slight swaying forward and backward by the photographer or the subject after focus lock and before exposure. The more recomposition, the longer the time between them so the more chance of sway.