I've thought about the field curvature problem but it doesn't explain
all. I had to manually focus the Kiron 28-105 and it performed quite
well at center and edges... just not as well as the Tokina.
Furthermore, since I was focusing exclusivly on the center, any field
curvature should have caused problems at the edge which is the reverse
of what I saw for the Kiron 28-70 and the Zuiko 28. Finally, if there
was a problem with altererd focus screen position it should have shown
up equally bad on all lenses when I was manually focusing. That's not
the case.
There's lots more testing to go so I'll pay more attention and see if I
can figure this out. Who knows, maybe these two lens samples just
aren't particularly good ones. I have rarely used either lens. The
28-70/4 Kiron is a rare bird snagged on the bay years ago and the 28/2.8
Zuiko came gratis along with an OM-2. I've had them both for years but
have almost never used them.
Chuck Norcutt
AG Schnozz wrote:
> Secondly, this focus issue you identify with the Tokina is
> probably the reason you are seeing poor center focus with the
> manual lenses. They are NOT flat-fielded, and I'm thinking that
> the actual intercepted plane of focus (which is curved with
> non-flat-fielded lenses) is farther out on the sensor. If you
> are experienceing an altered focus screen position (Others have
> complained of their 5D not manually focusing correctly either)
> then this would help explain this oddity.
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