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12-15-2011, 03:15 PM   #1
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K-x using 28mm/2.8 promaster with "A"

I'm trying to figure out how to use a promaster 28mm F/2.8 lens, that I acquired along with a promaster camera. It has an "A" setting on the aperture. I think the Promaster 28mm/2.8 is a Vivitar clone, based on the picture. I'm trying to use the lens on my K-x DSLR, but so far, the "A" setting doesn't work. It appears that the aperture always winds up being set to F/2.8, no matter what the K-x camera says, in the "AV/Tv/P" modes.

Manual aperture works ok in "M" mode. The +/- or the green button button will set the shutter speed correctly, which takes a fraction of a second while the camera makes a sound like its taking a picture. The green button is set to Tv Shift.

I don't have any other manual focus lenses with an "A" setting. I assume they should work just like autofocus lenses. My Tamron 28-75mm has an aperture ring, but of course I only use it on the "A" setting, which works correctly.
- Sheldon

12-15-2011, 04:12 PM   #2
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QuoteOriginally posted by sheld Quote
I'm trying to figure out how to use a promaster 28mm F/2.8 lens, that I acquired along with a promaster camera. It has an "A" setting on the aperture. I think the Promaster 28mm/2.8 is a Vivitar clone, based on the picture. I'm trying to use the lens on my K-x DSLR, but so far, the "A" setting doesn't work. It appears that the aperture always winds up being set to F/2.8, no matter what the K-x camera says, in the "AV/Tv/P" modes.

Manual aperture works ok in "M" mode. The +/- or the green button button will set the shutter speed correctly, which takes a fraction of a second while the camera makes a sound like its taking a picture. The green button is set to Tv Shift.

I don't have any other manual focus lenses with an "A" setting. I assume they should work just like autofocus lenses. My Tamron 28-75mm has an aperture ring, but of course I only use it on the "A" setting, which works correctly.
- Sheldon
Could be a stuck aperture? If you set it to the smallest aperture in-camera (~f22 – maybe) and look directly on the lens and press the green button while in manual mode, can you see if the aperture blades are actually stopping down? You can also try and manually operate the aperture lever on the lens and see if the blades move smoothly, or at all. I think you’re getting the correct exposure in Manual mode because your're setting the camera to meter at the aperture the lens is stuck on by using the green button. Hope this helps!
12-15-2011, 05:07 PM   #3
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QuoteOriginally posted by sheld Quote
I'm trying to figure out how to use a promaster 28mm F/2.8 lens, that I acquired along with a promaster camera. It has an "A" setting on the aperture. I think the Promaster 28mm/2.8 is a Vivitar clone, based on the picture. I'm trying to use the lens on my K-x DSLR, but so far, the "A" setting doesn't work. It appears that the aperture always winds up being set to F/2.8, no matter what the K-x camera says, in the "AV/Tv/P" modes.

Manual aperture works ok in "M" mode. The +/- or the green button button will set the shutter speed correctly, which takes a fraction of a second while the camera makes a sound like its taking a picture. The green button is set to Tv Shift.

I don't have any other manual focus lenses with an "A" setting. I assume they should work just like autofocus lenses. My Tamron 28-75mm has an aperture ring, but of course I only use it on the "A" setting, which works correctly.
- Sheldon

The Promaster lens can't really be a "Vivitar clone", because Vivitar never manufactured anything. However, it could be identical to a Vivitar lens, because Ritz Camera (Promaster is their house brand) probably bought a lot of lenses from the same companies that made lenses for Vivitar.

First of all, does the lens stop down when off the camera? Set the aperture ring to f/16 or f/22 and take it off the camera. A k-mount lens will stop down to the selected aperture when off the camera. If yours does not, you've got a stuck aperture or a broken mechanism, or possible oil on the blades. Counterintuitively, oil on shutter blades acts more like a glue than a lubricant. Cleaning aperture blades is not a DIY project, except for the most brave.

Secondly, clean and examine the electrical contacts on the lens. I assume the ones on the camera are clean, because you said your other lenses work properly. A pencil eraser is good for cleaning contacts. Just make sure you do it away from the camera and clean any rubber "crumbs" off before re-mounting the lens.

Sometimes the contacts are tiny ball-bearings that are spring loaded so they can ride over the camera contacts as you mount the lens. Occasionally, the spring behind the ball gets broken or stuck and the ball does not protrude enough to make contact with its counterpart on the camera. I'm not sure what is involved in fixing this, if you can't get it to pop back out on its own, with a little poking and prodding.

And, yes, except for autofocus, an "A" lens should work exactly the same way as your other Pentax lenses.
12-16-2011, 02:32 AM   #4
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I tried cleaning the contacts with an erasor, but it didn't help. The lens works fine as a manual aperture lens though, and the lens definitely stops down when I change the aperture on the lens, because I can see the changes in both the DOF, and the shutter speed increases 2x per F-stop. But if I try to use the "A" setting, and set the aperture to F/8 internally, the photos are way over-exposed, because they're taken at F/2.8

It looks like one of the pins may be stuck, but I didn't compare to my other lenses.
- Sheldon

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