Comment at bottom.
At 10:03 AM +0000 1/23/02, olympus-digest wrote:
>
>Date: Wed, 23 Jan 2002 03:55:12 -0600
>From: "Jon Wichman" <jwichman@xxxxxxxxx>
>Subject: Re: [OM] OM lens fascination
>
> > accept other Zuikos you mentioned. May be one of the possibility is
> > focusing problem, the wide angles has to be focused very carefully,
> > don't expect DOF can cover slightly off focus, you need very sharp
> > focusing. To me only focus on the matte area can do, no split image!
>
>Good point there!
>Also, is the camera being moved or, are you shooting all of these from the
>same distance?
>
>With a large enough newpaper sample or samples, you should be able to test
>all of the lenses from the same distance which, I think is important.
>With the wide-angles, to fill the frame, you would have to be much closer
>and, they are harder to focus and, would have less depth of field.
>
>I think that you should choose the closest subject distance that all of the
>lenses
>can focus and, go from there. Or, at least choose a reasonable compromise.
>On the 35-80 that you liked so well, what focal lengths did you test it at
>and, did you zoom in to focus (of course you did - who wouldn't).
>Same with the 90mm. Easy to tell focus, far enough away to have some DOF.
>I think C.H. could be onto something here...
>
>If they are "soft" all over then, something is probably wrong with the
>testing procedure.
Or the camera is mis-adjusted, as was my original OM-1 in 1975. The problem
was that the mirror rest wasn't in quite the right place, so the viewfinder
focus (with mirror in place) and the actual focus (with mirror flipped up)
differed significantly.
One can test this at home:
1. Buy a small piece of thin finely-frosted ground glass. I got mine by mail
order from Edmund Scientific (<http://www.scientificsonline.com/Scientifics/>)
some years ago. Using a glass cutter and steel ruler, cut the glass into a
strip 35mm wide and perhaps 70mm long. With 220-grit wet-dry sandpaper face up
and wetted on a large sheet of glass, grind the cut edges of the ground glass
straight, and smooth any sharp edges or corners off.
2. Put the camera under test on a tripod, remove the back, and tape the 35mm
wide groundglass to the body where the film would go, with the ground side
towards the lens. It's essential that the glass be held tightly to the body.
(Tape residue can be removed with a kleenex dampened with lighter fluid,
without damaging metal or paint.)
3. Point the camera at a suitable test target. I used distant trees in
winter, as the twigs are very small and make good resolution tests. A
newspaper on the wall should also work.
4. Using the viewfinder, focus the camera. Using a hand magnifier, inspect
the image on the ground glass. (If this is done outside, use of a black fabric
focusing hood may be helpful.) Is the image on the groundglass sharp? More to
the point, does it get sharper if you re-focus, or is the viewfinder focus
position the best? If the viewfinder focus and the groundglass focus differ
significantly, it will prove impossible to make sharp photos with wide-open
lenses. The only fix is to have the camera cleaned and adjusted.
Joe Gwinn
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