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Unsymmetrical aperture on 55/4 lense


pavel_m.

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I have a Pentax 67II with two lenses - 55/4 and 200/4. On the 55/4 the aperture is not symmetrical octagon as nicely as on 200/4. It is in the middle but not symmetrical. Did anybody noticed the same problem, does it effect image quality ? Should I return that lens (is bright new) or it is not problem at all. I appreciate any suggestion on that.

Thank you.

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I looked into "Interchangeable lenses - operating manual" which I got

with lenses and found on page 9 following <A

HREF="http://www.top.cz/talema/pavel/temp/Aperture.jpg">picture of

aperture</A>. As you can see the aperture is not circular -

symmetrical, same way as main on 55/4. So maybe this can not be

perfectly circular due to construction of aperture, but aperture on

my 200/4 IS symmetrical. So what do you think ?

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Pavel, what I was talking about was, in a perfect world diaphragm

openings should be circular to have the least diffraction possible. I

realize that yours has eight blades and forms an octagon. The better

lenses have more blades to try to accomplish a circular opening. It is

not possible to do it at all apertures in any lens I've seen, although

the 600mm 12 blade diaphragm forms a circular opening at several

apertures. Your oblong diaphragm opening will increase your

diffraction but the difference between an octagon and what you have

may not be easily detectable in your slide/print.

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  • 2 weeks later...

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