Unsymmetrical aperture on 55/4 lense

greenspun.com : LUSENET : Pentax 67 SLR : One Thread

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.

-- Pavel Medek (talema@top.cz), March 15, 1999

Answers

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

-- Steve Rasmussen (srasmuss@flash.net), March 15, 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.

-- Steve Rasmussen (srasmuss@flash.net), March 17, 1999.

I looked into "Interchangeable lenses - operating manual" which I got with lenses and found on page 9 following picture of aperture. 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 ?

-- Pavel Medek (talema@top.cz), March 17, 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.

-- Pavel Medek (talema@top.cz), March 30, 1999.

Moderation questions? read the FAQ