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10-05-2013, 03:12 PM   #1
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Sigma 28-70 f/2.8 manual focus aperture problem

I recently acquired a Sigma 28-70 f/2.8 manual focus lens.

It has an "A" setting on the aperture ring, but when I set the camera (K-x) to Av mode, and turn the aperture setting on the camera, it won't go below f/5.6 (most focal lengths or f/4 (at 70mm). I can't set f/2.8 by the camera.

If I use the aperture ring to set aperture, I still get correct exposure, but the camera still won't report an aperture wider than f/5.6 (or f/4 at 70mm).

Do anyone know if this is a quirk of this lens, or an indication that something is out of alignment?

This lens also has a little bit of the "old Sigma" stickiness in a few spots. Does anyone know a solution for this? It's not a huge deal, because it seems like it doesn't actually come off on your fingers (so you won't get anything else dirty because of it).

One other thing about this lens. I think the hood mechanism is really clever, although it's probably not unique to this lens.

The lens is shortest at 70mm and longest at 28mm, but only the very end of the lens moves.
The hood connects further down the body, so it doesn't move.
This means that you have a short hood at 28mm and a longer hood at 70mm.

The downside, and the reason they probably didn't use this design in later lenses, is that the lens hood is really fat. It makes the lens huge when you stow the hood by reverse mounting it.

I've just realized I have about a dozen Pentax film cameras sitting around, so I'll go try out the Av setting on a few of those, and see if it works correctly on any of them. I don't expect it to, but if it does, I'll update this post.

UPDATE: I put it on my PZ-70, and it worked perfectly. That's confusing.
Next, I put it back on the K-x, and it was back to thinking it was f/5.6. So I went digging around in the menus, looking for something that would explain this issue, and changed a few settings I hadn't realized were set (but probably aren't important). After exiting the menu, the display magically said f/2.8! I played with it in P and Av modes and it was now happy with f/2.8 throughout the focal range, although an occasional f/4 would show up and then immediately disappear.

I will say, I turned the camera on and off several times before, and even changed the batteries, and it had no effect. Does anyone know if changing settings in the menu resets something that doesn't normally reset with just "off and then back on"?

UPDATE 2: I just went and cleaned the contacts (as this was suggested for a similar problem in a different thread about a different lens), and played with it a bit more. It seems has to do with the exact position of the lens when mounted. If I twist it a little bit (after it's locked on to the mount), I can change the max aperture value, in the range of f/2.8, f/4, f/5.6. So it seems like a mechanical problem. Is anyone familiar with this? Can it be fixed with a modification or adjustment to either the body or lens?

Last edited by thornburg; 10-05-2013 at 03:40 PM. Reason: Update!
10-06-2013, 01:22 AM   #2
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Is it the constant f2.8 or is it the f2.8-4?
10-06-2013, 05:09 AM   #3
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It's the f/2.8 constant.
10-06-2013, 08:10 AM   #4
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I think it's a contact problem. If you look at how the KA contacts work to transmit aperture, you should be able to figure out what's going on:

Features and Operation of the Ka Mount

10-06-2013, 05:32 PM   #5

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I agree with nater.

To be a little less cryptic, you need to find the closest match to the aperture range of your lens, then make sure that the correct pattern of contacts are shorted (conductive) or insulated. I've had to do this with at least two Sigma AF lenses (EX 300 f2.8 APO and EX 180 f3.5 APo DG Macro) to get them at least close to transmitting the correct fstop to the camera for exif purposes when used with the Pentax F 1.7x AFA. I've used very thin strips of Scotch Magic transparent tape to insulate the proper contacts with good results.


Last edited by snostorm; 10-06-2013 at 09:14 PM.

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aperture, av, bit, camera, f/2.8, f/4, f/5.6, hood, k-mount, lens, pentax lens, slr lens
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