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

Subject: Re: [OM] diffraction, aperatures, f-stops, extensions
From: frieder.faig@xxxxxxxxxxxxxxxxxxxxxxxxx
Date: Fri, 8 Mar 2002 20:27:59 +0100
On Thu, Mar 07, 2002 at 02:21:08PM -0600, Dean C. Hansen wrote:
> Dreammoose recently wrote:

>     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?

It`s the challenge of extreme macro-Photography.
The Zuiko 20mm macro is on the limit, what one can do with 
a macro lens. Otherwise a microscope is necessary for magnifications 
like that, and greater.

>     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?


Yes twice, 
I`ve a book about OM-System. In the Macro chapter is a d.o.f.-list
depending on magnification-ratio with the smallest recomended 
apperture. Usually it is the apperture where the circle of confusion from 
diffraction matches the 0.03mm used for d.o.f calculations.

Here is the list:
Magn.   smallest recomended apperture
  1x            16
  1.5x          11
  2.5x          11
  3x            8
  4x            8
  4.5x          5.6
  6x            5.6
  7x            4
  8x            4
  10x           3.5
  16x           3.5  (my guess: list was intended for the 3.5/20 Macro)

With the 20mm lens and 270mm extension the magnification ratio is appprox. 13x, 
so with F8 you`re already beyond the recommendations. 

>     I have a lot to learn about optics--and the ways of this 20mm macro
> lens.  Anyone have any suggestions for using this guy?

Practice to find the optimum compromise. The values from above can be used
as guide, but not cemented law. My macro experience so far is limited to about 
1.5x.
 
Frieder Faig

for German readers:
Link to a description of the book, the table is from (Ia there a english 
version?):

http://www.stud.uni-hannover.de/user/73239/OM-Buch/OM-Literatur.htm#Olympus

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