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[OM] 250mm F2 aperture stops down slightly even wide open?

Subject: [OM] 250mm F2 aperture stops down slightly even wide open?
From: Dawid Loubser <dawidl@xxxxxxxxxxx>
Date: Wed, 5 May 2010 22:12:25 +0200
Hello fellow Zuikoholics,

Could anybody that owns / uses a 250/2.0 please check something for me  
on your lens?

I noticed something odd today - when the lens is set wide open, and  
you look into it
while pressing the DOF preview or tripping the shutter, the aperture  
blades stops down
a little bit (unlike any other OM Zuiko that have used).

I know object scales when looking into a huge piece of glass like this  
is deceiving, but
it "appears" as if the aperture closes down by a radius of 5mm  
(perhaps less) - a small fraction
when compared to stopping down to f/2.8, but still visible.

I was just curious whether it's a quirk of my lens, or whether this is  
normal. The only other
lens I've seen with similar behaviour, at certain focus distances, is  
the electronic
Canon EF 100mm f/2.8 Macro, but I know there are others (like the CV  
125mm Macro).

regards,
Dawid
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