At 07:17 AM 12/21/2008, Chuck-the-man wrote:
><snip>
>
>I don't understand why it can vary with the lens but I have often
>wondered whether "coasting time" is a problem. That is, when the focus
>motor is commanded to stop moving it doesn't stop instantly but instead
>maybe continues turning for another millisecond or two or three. That
>would mean the camera has to anticipate the approach of best focus and
>shut off the motor before best focus is actually achieved. This is
>purely uninformed speculation on my part. I have no idea what really
>causes focusing errors.
As C.H. alludes to, this should not be a cause as the auto-focus is
a servo feedback system. Only if you take the shot by fully pressing
the shutter release, such that the system fires as soon as it get AF
indication, should focus over or under shoot be a problem.
More likely there is something fooling the AF sensor itself that
has to be compensated for, or the compensation algorithm has
a bug for particular lenses.
I remember AG Schnozz's discussion on bokeh with zoom lenses in
the past. All that lens correction can make for weird effects in out
of focus regions. I have seen sharp elements in the background
bokeh. My guess is such lens out-of-focus artifacts are a more
likely culprit in fooling the AF correlation algorithm.
One of the reasons I like a good viewfinder and manual focus override.
WayneS
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