aperture issue - past f5.6 drops to f22 and won't come back!

[ramcewan]

I bought the 24mm f2.8 Sigma Super-Wide manual focus lens in AR mount. I have had it for just about a week and I really liked it but then yesterday I went to use it and found I couldn't focus at all and the picture was dim in the view finder. Upon further examination I found the aperture had stopped itself down all the way!

So I took the lens off and tried the aperture ring and it didn't move. I moved the little aperture control lever on the inside of the lens and the aperture opened back up and then returned to where it was supposed to be (f2.8 at that point).

With the lens off I tried moving it from f2.8 to f4 and that was okay then f4 to f5.6 and it snaps to f22!

I looked with a bright light and don't see any evidence of oil or anything else on the aperture blades.

Not sure what to do about this. The seller is offering me a refund so I guess I will take it but I feel bad cause the seller bought the lens from someone else who sold it as a nikon mount and the seller didn't even try the lens before reselling it to me.

It also sticks open when stopping down, with light tap on the side causing the aperture to snap into place and sometimes the aperture shape is miss formed.

[iangreenhalgh1]

I have seen lenses do similar things, I didn't try to fix them though so don't know what the problem might be, sorry.

[mdarnton]

I was thinking of extending the close-focus range of my 28/2 Kiron until I noticed the following: on the Nikon version, the aperture stop-down mechanism consists of a large ring inside the back of the mount. One tab faces backwards out the back of the lens, for the camera to move. Inside the lens a similar, but very long, tab reaches down towards the front of the lens. The spring-loaded stop-down mechanism on the diaphragm itself rides against the long tab, sliding up and down it as you focus. When I tried to extent close focus, I discovered that this long tab extended ONLY far enough to catch the tab on the diaphragm that rode forward and back with the glass during focusing up to the lens' normal close focus. One bit closer with the focus, and the two lost contact, and the diaphgram slammed shut. The only way to get them back together was to turn to the smallest f-number, and bring the focus back into the normal range, with the two arms having regained their normal contact.

Short version: there's a finger in the diaphragm system that's just as long as it needs to be, and it can lose contact with similar results to what you describe.

Perhaps in your situation something is loose enough that one of these parts wobbles out of line and contact is lost (the same effect as my too-close focus, but a different cause, sidways loss rather than lengthwise). If you aren't the kind of person who digs into things yourself, you won't be able to fix it.

The end of the story of the Kiron was that to extend focus all I had to do was pull a screw. I put the screw back in, and now it's back to the way it was born.

[ramcewan]

mdarnton - I think I follow what you are saying and notice the focus distance seems to make a difference. Focus to infinity and the problem seems worse. Macro focus and the problem seems to be less problematic. But overall the more I play with it the worse it seems to get - either sticking as it stops down or staying shut as it is opened up.