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Re: [OM] ( OM ) lens resolution capability

Subject: Re: [OM] ( OM ) lens resolution capability
From: Winsor Crosby <wincros@xxxxxxxxxxxxx>
Date: Wed, 16 Oct 2002 09:20:24 -0700
Hi folks

Wayne Harridge wrote

What seems a bit strange is that the corner resolution is better than the centre for many of the tests - I wonder if there is some sort of focus problem with the
 camera (e.g. miiror or screen alignment problem).

I agree. I think that probably I didn't have the camera and target board
perfectly parallel.

Also it is **not easy** to read those chart results - not at all.

Later I read in the instructions (like, when all else fails, read the instructions
<g>) that a sharper image can be obtained on the screen if one makes a pin-
hole (as in "very big pin-hole" ) mask and places it over the projector lens.
Stands to reason - even if the projector lenses ARE designed to be sharpest
at full aperture, it wouldn't be surprising if a little stopping down improved
what was projected.

HOWEVER, if it is SO difficult for me to obtain a very sharp image on fine-
grain 50 ASA slide film when the camera is weighted firmly on the floor and
the mirror-up & pre-fire aperture routine is done (even if the two-dimensional
plane subject is slightly off parallel), what chance of getting a sharp image
with a hand-held camera and a subject that is three-dimensional?

Brian - somewhat puzzled on this topic as usual.

Only macro lenses are designed to have perfectly flat fields. The center and the corners of the film plane have different focus points with an ordinary lens, but the center is usually the sharpest. If your best focusing efforts produce sharp corners and soft centers(sounds like a chocolate) your focusing system may be off. The camera may need servicing to make sure a focused image on the focus screen corresponds to sharp center focus on film plane. The angle of the mirror could be off. And other things I do not know about. It would explain the soft slides overall as well if a carefully focused viewfinder is producing a focused image in front or in back of the film plane.
--
Winsor Crosby
Long Beach, California


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