[Roam Redway]

I took out my film era Sigma Mini-Wide II 28 mm, f/2.8, MF lens. It has an “A” setting on its Aperture ring.
I put it on an old K-X I keep around.
In Aperture priority, no matter what the ISO or shutter speed, I cannot get it to open wider than f/5.6.
(Yes, I have set the K-X to allow use of Aperture ring. I can turn the ring to f/2.8 but have no way of knowing whether it’s actually opened all the way up.)
It must be a lens problem. When I put on my trusty old 50 mm, f/1.7 that lens opens all the way up.
This is not a big deal; I have a newer AF 28/2.8 that works just fine. I just like my old glass and hate to see old lenses wither away.
I’m just curious as to what might cause it and whether there’s anything I could do about it.
Thanks in advance.

[kypfer]

First thing to ascertain is whether the aperture mechanism functions across it's full range when it's not fitted to a camera ... it's possible the blades have got sticky. Then I'd check the electrical contacts are clean, give them a good polish with a soft dry cloth. Then I'd try it on another body, just in case there is something weird wrong with the K-X.


Good luck

[Roam Redway]

kypfer:
Thank you for your reply. Per your suggestions I have:
- verified that my aperture will fully open manually;
- tried the lens on my K-5 with the same result (i.e., won't open beyond f/5.6 usign "A" setting);
and
- tried buffing the contacts.

Same results.
I'll probably just have to live with it or shoot by turning the aperture ring if I want the lens to open wider than f/5.6.
The aperture ring is difficult to turn. I may take the opportunity to use this old lens to do a partial disassembly and try to fix the ring.

[kypfer]

kypfer:
Thank you for your reply. Per your suggestions I have:
- verified that my aperture will fully open manually;
- tried the lens on my K-5 with the same result (i.e., won't open beyond f/5.6 usign "A" setting);
and
- tried buffing the contacts.

Same results.
I'll probably just have to live with it or shoot by turning the aperture ring if I want the lens to open wider than f/5.6.
The aperture ring is difficult to turn. I may take the opportunity to use this old lens to do a partial disassembly and try to fix the ring.

Hmmm ... sounds as though the aperture operating lever on the lens may be bent, so the camera simply can't push it far enough. I have read about (but never experienced) some older lenses where the "guard" for the aperture lever on the lens does interfere with the operation on some modern cameras, but how that manifests itself I don't know. Unfortunately I don't have a similar lens to compare notes with

[Just1MoreDave]

kypfer:
Thank you for your reply. Per your suggestions I have:
- verified that my aperture will fully open manually;
- tried the lens on my K-5 with the same result (i.e., won't open beyond f/5.6 usign "A" setting);
and
- tried buffing the contacts.

Same results.
I'll probably just have to live with it or shoot by turning the aperture ring if I want the lens to open wider than f/5.6.
The aperture ring is difficult to turn. I may take the opportunity to use this old lens to do a partial disassembly and try to fix the ring.

You might be able to figure out if this is an electrical problem or mechanical. When you set the aperture ring on the lens to A and the lens is not on the camera, the blades should be easy to see and the opening should be very small, maybe a bit larger than a pinhead. Watch the blades when you mount the lens to the camera. At a certain point when rotating the lens to mount it, the blades should start to open, and fully disappear when the lens is locked on the camera. That is to make focusing easier, because it lets the maximum light into the viewfinder. If your lens doesn't work like this, it's something mechanical, like a bent aperture lever. Compare the lever with another lens that works correctly. You can often tell when a lever doesn't look right.

The electrical contacts have an exterior surface you can see and clean, but also an inner contact, spring, and another contact, then internal connections. At least some of the Mini-Wide IIs have an additional pin for Ricoh program cameras. You can take the mount off and clean everything and it'll probably fix it. Try this really easy diagnostic test first, though. (I will feel like a genius if this works.) Set the aperture ring to A. Put the lens on the camera. See what the smallest possible aperture you can set with the camera controls (e-dials). If you can turn the e-dial and see f45 on the display, just one of your contacts has failed to conduct electricity. That failure has caused your camera to see the lens data as a lens with an aperture range of f5.6 to f45 instead of f2.8 to f22. The failed contact is one of the pins next to the A pin. Some of my Sigma lenses have a brass metal slug pushed into the chromed mount here. The contact system and codes are fully explained here. You can see in the chart that the difference between an f2.8-f22 lens and an f5.6-f45 lens is just one contact.

Features and Operation of the KA Mount | The K-Mount Page