Using the new 75mm f/2.8 I noticed that the f/stops displayed in the P67 II's viewfinder differ 1/2 stop from the aperture setting on the lens, especially between f/2.8 and f/5.6. Less noticeable my 165mm f/2.8: only when stopped down to f/22 the AE finder indicates a flickering f/27. No problems with the 45mm lens.

Has anyone similar experiences? Is this an AE finder phenomenon, or could the brand new 75mm's f/stops be so different to the exact aperture?

By the way, I am quite happy having found this site ...

-- Michael Wanner (mwfoto@gmx.de), January 26, 2002

Answers

Jim, I tend to agree with you on this, especially since the link between the diaphragm and readout is not mechanical. Many wide lenses will have slightly better light transmission in the center vs the marginal areas. This could cause the display to read differently when the lens is wide open vs stopped down. The use of SMC increases the light transmission at each surface to 99.8%. But, the low index elements don't require a seven layer coating to prevent reflection. So my guess is that Pentax may only use two layers on these elements. The bottom line is that each lens transmits light differently with some being better than others. The 300 Takumar is one of the worst, even though it is SMC.

-- Steve Rasmussen (srasmuss@flash.net), January 30, 2002.

Michael,

using the 2.8/165mm lens at f/2.8, I also obtain a 1/2 stop deviation from the actual aperture in the P67II viewfinder. I have no problem with the 2.8/90mm lens, so this is not a general problem of fast (f/2.8) lenses.

It's bad news to hear that this problem remains at f/5.6 with the new 75mm lens. We should make sure that Pentax is aware of this problem. I think it is caused by the aperture readout mechanism. It is definitly not a problem of inaccurate aperture openings.

Welcome to this forum (if I may dare to say so, because I was absent quite a while)!

-- Joachim Inkmann (joachim.inkmann@gmx.de), January 29, 2002.

This may not be a problem per se. The physical f-stop ratio may not actually represent the actual light transmissability (Steve, is that a word?)of the lens. The meter my be giving you the correct effective aperture, which is somewhat smaller than the markings on the aperture ring. The number of elements in a lens will determine how much light is lost between the front objective and teh film plane (and meter). The more elements, the more light loss. This is common on 35mm zoom lenses and electronic cameras. The meter will tell you the real story about your lens. Therefore, if my theory is correct, the "problem" is not with the camera or lens, but a problem with physics!

My $.02USD

JFK

-- Jim Korczak (korczaks@ptdprolog.net), January 29, 2002.

Hi Michael,

I had the same problem with my 200mm f/4.0. The Pentax technical service fixed it by adjusting a pin on the lens itself. Of course I don't know whether this is possible with other lenses. It is definitely NOT an AE finder phenomenon. I hope this will help you. Greetings, Peter van Raak.

-- Peter van Raak. (pvanraak@hetnet.nl), May 11, 2002.