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[OM] diffraction, aperatures, f-stops, extensions

Subject: [OM] diffraction, aperatures, f-stops, extensions
From: "Dean C. Hansen" <hanse112@xxxxxxxxxx>
Date: Thu, 07 Mar 2002 14:21:08 -0600
Dreammoose recently wrote:
    "The problem is that light does funny/interesting things when it
passes
through small holes, it spreads out, rather than going straight."
    In short, if you try to get maximum depth of focus by going to a
high f-stop, you lose resolution.  The culprit, as I understand it, is
diffraction.
    O.K., here I am, trying out my new 20mm f2 OM auto-macro lens. It's
mounted on the OM bellows, with the OM4T on the other end.  The whole
set-up is pretty solid, and I'm using the delayed shutter release to
mimimize vibrations from the mirror retracting up. Focusing screen is
the cross-hairs/clear field one, the 1-12.
    I've tried to eliminate any instability in the whole set up, and I'm
using a single T32 to take shots of the scales on butterfly wings.
    When I close down to f8, the opening in the diaphragm is clearly a
hexagon about 3mm across.  With the bellows cranked out, the front of
the lens is about 27cm, or 270mm, from the film plane, so isn't my
effective f-stop 270/3=90?  Or is it?  Is this a concern?
    Whatever, I'm having trouble getting sharp photos.  One thought is,
am I stopped down too much?  O.K. again, the solution is to try a series
of shots at wider apertures;  I'll do it.  But does diffraction become a
greater and greater problem at longer extensions?  The aperture of the
lens wide open measures only 8mm, so at a distance of 270mm lens-to-film
plane, is my f-stop 270/8= 34?  Is that bad?
    I have a lot to learn about optics--and the ways of this 20mm macro
lens.  Anyone have any suggestions for using this guy?
Many thanks,
Dean


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