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Backfocus again - here's what's wrong

 
 
Steve
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      02-09-2008
The problem seems to be that the AF-S motor in the lens doesn't stop
where the camera wants it to. But the camera says "close enough" and
confirms focus. I think it's a problem with this lens because it
seems to be worse now than even just a few days ago and the more I use
it today testing the focus, the worse it's getting. I think the AF-S
motor is going bad and I'm going to send it in for service.

When I focus the lens manually and use the viewfinder focus indicator
to determine when it's in focus, it's spot on perfect. With a non-AFS
lens where the camera has positive control of focus with it's own
motor turning the lens screw drive instead of a seperate motor in the
lens, the focus is almost spot on. With the Sigma 70-300 non-AFS
lens, I've seen variations using the screw drive AF of being anywhere
from spot on to about 2mm front focus. That's probably due to the
tiny bit of slop in the screw drive and I can easily live with that.

Steve
 
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John Navas
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      02-09-2008
On Sat, 09 Feb 2008 08:47:21 GMT, Steve <(E-Mail Removed)> wrote in
<(E-Mail Removed)>:

>The problem seems to be that the AF-S motor in the lens doesn't stop
>where the camera wants it to. But the camera says "close enough" and
>confirms focus.


Most cameras don't even check ASAIK, more concerned with focus speed
than with focus accuracy, depending on depth of field to cover the
error. My own experience is that manual focus will usually be
noticeably more accurate than even the better dSLR focusing systems,
with certain lenses at least (although there may be better ones that
I haven't tried).

>I think it's a problem with this lens because it
>seems to be worse now than even just a few days ago and the more I use
>it today testing the focus, the worse it's getting. I think the AF-S
>motor is going bad and I'm going to send it in for service.


That sounds quite reasonable.

--
Best regards,
John Navas
Panasonic DMC-FZ8 (and several others)
 
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