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01-21-2017, 03:38 AM   #1
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Strange problem with Tamron lens aperture

Hi

I got a second hand Tamron 28-75 f2.8 in almost new condition. But after trying it out, it seems to have a strange problem with it's aperture (and hence the metering). On wider zoom ranges (towards the 28mm) after taking a single shot, the aperture seems to close down halfway, the viewfinder goes darker. This affects the metering in all the following shots and they are heavily overexposed. Initially, I thought it was the problem of a sticky aperture. But on examining the aperture lever and the blades closely, they seem to be in perfect condition, no oil and quite snappy. The problem is strange because the the aperture behaves perfectly when the lens is zoomed out (on or beyond 50mm say). It is only at wider zooms that the aperture behaves in this manner.

Has anybody faced such a problem before, or does anyone have an idea as to what the problem might be? Any help is highly appreciated.

Thanks

01-21-2017, 03:55 AM   #2
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What camera are you using? There was a similar problem with K-30 and K-50 bodies that comes out with age, not shutter count. There was some method that managed to get the aperture lever unstuck but not for everyone.
There was an article too about the problem here
01-21-2017, 07:51 AM   #3
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Does it do this with other lenses? If not the lens probably needs servicing.
01-21-2017, 08:27 AM   #4
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QuoteOriginally posted by paulzhere Quote
Has anybody faced such a problem before, or does anyone have an idea as to what the problem might be? Any help is highly appreciated.
QuoteOriginally posted by Sterby Quote
What camera are you using?
As Sterby suggests, your experience may be related to your camera. Your profile indicates that you shoot with a K-50. If you are still using that camera, it may be the source of the problem. There is a known problem with both the K-30 and K-50 that involves aperture control.

You can evaluate the lens off-camera by working the aperture actuator by hand. If the action is smooth without binding or hesitation, it is unlikely that the problem is with the lens.

You can evaluate the aperture mechanism on the camera by setting the mode to "B" and releasing the shutter while looking into the front of the lens at various apertures. If the aperture does not fully open after releasing the shutter, you have problems. Similarly, if the aperture does not stop down appropriately (incrementally smaller at larger f-numbers), things are not right.

If there is a problem with the body, warranty service would be your best option. If the camera is no longer under warranty, a repair by the Ricoh/Pentax authorized service center will likely cost between $200 USD and $250 USD. Other options have recently been suggested, but I have no personal experience and cannot recommend.


Steve

01-21-2017, 11:51 AM - 2 Likes   #5
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[SOLVED]

Hi guys

Thank you all for your replies and suggestions/supports/links. It seems like I managed to fix the problem. Since the camera behaves perfectly with my other lenses (btw, @stevebrot, I use a K-70 now. I recently `upgraded' from my K-50), I was kinda sure that it must be the problem with the lens. I decided to do a little DIY and take the lens apart. I removed the lens mount and inspected the aperture lever. I initially thought that tightening the lever by sliding the screws to the right might do the trick. That would give the spring a slightly higher tension allowing it to bring the aperture diaphragm back with greater authority. So I did that and fixed the mount back. A stranger thing happened. The aperture lever seemed to get stuck and wont move at all. I unscrewed the mount once again and lifted it ever so slightly and tried pulling at the lever again. This time it did move but seemed to be held back by something. There was a lot of friction and I could hear some rubbing noise as the disc moved back and forth. Maybe the lever was rubbing against the lens assembly? I took out the lever arrangement once again, put it back in its original position and inspected it. Indeed it was looking a bit bent. I applied some force and straightened it ever so slightly. Put the assembly back and reassembled the mount. The lever was snappy again. But was the original problem solved? I mounted the lens on my K-70 and took a few shots. Voila! It seemed to work perfectly.

Now, as I read these posts I realize that @dcshooter was bang on. It was the bent aperture lever that was the problem. Thinking back, I realize that it is kinda logical. When the lens was zoomed out, the aperture lever used to rub against the lens elements much more then when it was zoomed in. That explains the strange issue!

Thanks again

Last edited by paulzhere; 01-21-2017 at 12:01 PM. Reason: [Solved]
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