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Re: [OM] 20 macro

Subject: Re: [OM] 20 macro
From: PCACala@xxxxxxx
Date: Thu, 29 Apr 1999 16:11:51 EDT
James writes:

> I want to photograph objects about 1-3 mm in size and have been told that a 
>  20 mm macro is the lens of choice.  

With the Auto Bellows you can get up to 16X magnification.  That isn't good 
enough for objects 1 mm to about 1.9 mm wide, though  You need a microscope 
for those, or shoot them at less than full frame and enlarge more (a good 
microphotographic technique for getting better depth of field).

> I assume the f2 (vs. f3.5) is the 
> correct lens.

Yup, for the auto diaphram model it is.  The older 20mm f/3.5 preset will 
also work, but wide open it already shows the effects of diffraction at f/3.5 
!!  Since you can't stop down much at all at these magnifications - with a 
limiting aperture of f/2 for 14x and 16x and f/2.8 for 12x and 10x - the auto 
diaphram isn't much needed.

>  Any suggestion where such a lens (used) can be 
>  obtained in the USA?

>From anyone who stocks Olympus equipment.  Just special order it.

Gary Reese
Las Vegas, NV

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