pavel_m. Posted March 15, 1999 Share Posted March 15, 1999 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. Link to comment Share on other sites More sharing options...
steve_rasmussen Posted March 15, 1999 Share Posted March 15, 1999 The disadvantage of having a non-circular diaphragm opening is that it will cause more diffraction. You probably would not be able to tell the difference, but if I were in your situation I would demand a product that met blue print specs exactly. Isn't that what we pay for? Steve Link to comment Share on other sites More sharing options...
pavel_m. Posted March 17, 1999 Author Share Posted March 17, 1999 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 ? Link to comment Share on other sites More sharing options...
steve_rasmussen Posted March 17, 1999 Share Posted March 17, 1999 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. Link to comment Share on other sites More sharing options...
pavel_m. Posted March 30, 1999 Author Share Posted March 30, 1999 Just to close the discussion. I asked the Czech Pentax Representation for their opinion. They confirmed that the aperture is within the tolerance, so they can not replace it. Maybe I am so pedant. Link to comment Share on other sites More sharing options...
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