The effect of diffraction will be more noticible the
farther back the film plane is from the lens.
however, this should not be an issue for you since you
are using F8. Diffraction only comes in to play at
around F22 (I believe, please anyone correct me if I
am wrong.) This is part of why it is quit rare to see
lenses with more then F22 stop.
F8 should not be giving you any problems.
Something else. I imagine that the scales you are
taking pictures of are pretty flat. You should be able
to open up the stop more then F8 and still get a good
picture, provided the subject is parallel to the film
plane.
Also your effective FStop is not directly realted to
the effect of difraction. The only thing that effects
that is the size of the aperture. The effect will be
more noticible farther away, but only becuase the
light will have spread out more, not because there is
effectivly less of it.
Effective F-Stops are only used for figuring exposure
time.
Date: Thu, 07 Mar 2002 14:21:08 -0600
From: "Dean C. Hansen" <hanse112@xxxxxxxxxx>
Subject: [OM] diffraction, aperatures, f-stops,
extensions
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
__________________________________________________
Do You Yahoo!?
Try FREE Yahoo! Mail - the world's greatest free email!
http://mail.yahoo.com/
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|