Re:F100 focus problems? [v04.n290/10]

[F. Javier Heredia]

Hi Toine
Hi Chris,

I'm suffering from the same problem. I bought my F100 last January
here in
Switzerland.
When using a Sigma 70-300mm and focussing at 70mm, the subject
is not sharp
when zooming to 300mm. If I focus at 300mm (the subject is now
sharp) and I
zoom back to 70 and focus again, the camera re-focuses and the
subject
become unsharp again.
If I have understood properly your problem, I guess this is not a problem
of the F100's AF, but of the zoom lens (which actually I suspect
it is not a
"zoom", but a "variofocal" lens). Up to my kwnolegde, a real zoom
lens  will keep the focus while zooming, which this is not
the case of
all the so called "zooms" lenses. I suspect this is the case of
your Sigma
"zoom". BTW, lets analyse your "experiment": the F100 locks focus
at 70 and
then, when you zoom up to 300, you see in the viewfinder a defocused
image: the body has no any relation with this lost of focus, as
the only
action you performed was the modification of the optical configuration
of
the "zoom", the F100 has not been asked to refocus, so there hasn't
been any
interaction between the body electronics and the lens, this is
just
the objective which has change the focus plane while zooming from
70mm to
300mm. This is my modest explanation of your problem.
This is exactly conform the article in the latest
issue of Chasseur
d'Images.
I have the article (not in front of me right now), but I thinkthe problem
was not the same. I will re-read it later.
By the way, the problem doesn't occur when using
my Nikon 35-105mm.
 
Maybe the optical design of the Nikon 35-135 is a true zoom, whichconserves
the focal plane while zooming.
BTW, this enforce the hypothesis that this is a lens dependent
problem, not an AF problem of the F100.

Best regards

Javier

--
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 F. Javier Heredia                
:
 Statistics and Op. Research Dept. : e-mail : [EMAIL PROTECTED]
 Univ. Politecnica de Catalunya    : phone.
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: FAX    : +34-3-401-58-55
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