Well easy for you to say having Dr.Focus as an in-house
consultant--or perhaps you consulted with his long lost cousin
Dr. Otto Focus.
I am no injuneer, but as I understand it CD AF uses an iterative
process and can't really determine if the lens is front or back focused
or by how much--that is why it is slower. I know the micro-adjust on
5D11 applies to the PD AF but I have no idea if that applies to CD at
all--perhaps not. With PD AF I would presume manufacturing tolerances
result in either the camera measuring the phase difference slightly
wrong, and thus sends a focusing command to the lens that's slightly
off, or the lens executes the focus command a bit too short or too
long, with a similar result. Or both things happen, either making it
even worse or actually correcting each other a bit. Why the
micro-adjust would no longer be correct on the extreme ends of a zoom,
I have no clue, but that is unfortunately the case. Magnified LV of
the Zuikos leads to precise focusing but a bit slow--- or even better
use a proper focusing screen on OM.
Mike (sometimes known as Dr. Mel Focus)
I frankly don't understand why there is any error at all. I would
think
that when the camera decides that the contrast is at maximum that it
simply orders the lens to stop moving. Perhaps they tend to coast
beyond the point where they should have stopped. Maybe my Tokinas and
Tamrons work well since they don't have high speed ultrasonic focusing
motors. Maybe they can't go fast enough to coast on by the right spot.
:-)
Chuck Norcutt
> Yes, but....
> With a poorly matched ZOOM/body, correction may not be accurate at
both
> ends of the zoom--must settle for sort of OK at both ends and spot
on
> focus in the middle.
> Why is that acceptable for an "L" zoom? That was my point but I did
> not articulate that clearly enough.
>
> Mike
>
>
>
>
>
>
> Canon has adopted that in the 5D Mk II and other cameras but,
> unfortunately, I have a 5D Mk I. The 5D Mk II supports custom focus
> data for 20 different lenses. But I doubt that it would work with my
> Tokina and Tamron lenses. Fortunately, I'm not aware of any focusing
> errors with those lenses.
>
> Chuck Norcutt
>
--
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