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[OM] Focus discrepancy in OM-4T: Solution sought

Subject: [OM] Focus discrepancy in OM-4T: Solution sought
From: Dogbreath <hopi@xxxxxxxxxx>
Date: Fri, 29 Sep 2000 10:23:08 -0500
Hi.

This 4T body has an annoying misadjustment in the focus. The plane of sharp
focus as revealed in the viewfinder does NOT match the plane of sharp focus
as rendered on the film plane. To give you an example, if I photograph a
flower close up and focus on a single pistil and make the exposure, the
processed slide invariably reveals that a DIFFERENT pistil is in focus -
one that is actuallly slightly more distant from the one I focused on.

Because so many of my macro slides were turning out this way, I thought I
should test the camera in this regard in a controlled setting. To do this,
I placed business card in a clothes pin and, with the pin as a base, stood
the card on its side. I then arranged the card at an 80 degree angle to the
film plane of the camera (held securely on a macro rail on a tripod) and
focused the lens near its close focusing limit, filling most of the
viewfinder with the card. The point of this was to create a test situation
where the depth of field would be as shallow as possible. I then moved
camera on the rail until only a single line of the tiniest copy on the card
was in focus. Lastly, I removed the camera back and placed a fragment of
ground glass against the film rails at the aperture. I compared the plane
of sharp focus as rendered at the film plane and it did not match - it was,
as my slides had told me - slightly more distant. I repeated this test with
all of my Zuiko lenses, and the results were the same every time (this was
no surprise to me because the misadustment would seem to be in the body).

My theory as to why this is happening is based on only a basic
understanding of SLR focusing. It would seem to me that the distance from
the lens mount to the film plane must match precisely the distance from the
lens mount to the focus screen. And because the light which travels to the
focus screen must first be reflected in the mirror, the position of the
mirror would be critical. Certainly, the position of the screen is also
critical, but as far as I can tell, the screen carriage is fixed and cannot
adjusted in a manner that would change the mount to screen distance. The
disheartening thing is that it's hard to see if the mirror can be adjusted
for this result, either. It doesn't really appear so. It would surprise me
if such an adjust could not be made, as I think we would all expect that
our cameras perform properly in this regard.

If any of you out there have experienced this problem and found a solution,
I'd love to hear about it.

Thanks for your attention.

DB



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