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06-17-2013, 06:35 AM   #1
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50-135 mount problem

My 50-135 suddenly seems to have a problem with it's mount. I hadn't used it for a couple of months after a journey to Surinam last autumn. I went for a shoot a couple of weeks ago and when I mounted the lens I had to use much more force than usual to turn the bayonet into it's place. After mounting it my camera (a K-5) went completely berserk, suggesting that there is a short circuit somewhere in the mount.

I went for a little investigation and found the following things:

- The lens behaves exactly the same on both my K-5 and my K10D
- Other lenses don't show this behaviour (tried 77ltd, 16-45, 18-135 and an old A400); they mount perfectly smooth and function like they should
- The lens mount doesn't show any defects or irregularities
- All shapes and distances within the lens mount are exactly the same as on other lenses
- When mounting the lens the bayonet starts giving more resistance when it's turned about halfway
- The lens is usable on my K-5 when I put it to MF and spotmetering, multisegment or centre-weighted make the camera go nuts

It seems to me like a mount failure like I said, but since there's absolutely nothing visibly wrong with the mount I'm quite puzzled. Anyone any suggestions?

06-17-2013, 07:53 AM   #2
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Sand in one of the button contacts that carries electrical and data signals from the lens to the camera is my best guess. Check to see if each of the gold color button contacts moves a bit as you press on them.

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06-17-2013, 08:00 AM   #3
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QuoteOriginally posted by Thijssie Quote
When mounting the lens the bayonet starts giving more resistance when it's turned about halfway
You note that 'all shapes and distances within the lens mount are exactly the same', yet the fact that there is resistance when turning it indicates that something is out of spec. What did you measure with? You are dealing with extremely close tolerances and areas difficult to measure.

Look for any scratches or marks on the area where the resistance occurs, you might be able to tell if something is bent. And I would try mounting a cheap extension tube rather than putting on the camera. You won't mar the camera mount and you can look through the tube and might be able to see if anything is binding. You can also try 'bluing' which is a dye that machinists use to tell where something is rubbing. A quick substitute is a magic marker, just mark up the area in question and then try to mount again. The areas that bind will rub off the marker. Again, do not do this on the camera use an extension tube.

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