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12-23-2008, 12:37 PM   #1
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70mm Limited over-exposing

Hi all-

I have a 70mm Limited which I love, but it does consistently over-expose by at least 2/3 of a stop, sometimes even over a whole stop. Whenever I put the lens on the camera I have to dial in exposure compensation to make up for it, and re-set it when a different lens goes on. Has any one else had this problem?

Does it sound like something which could get repaired?

This is on a K20D. None of my other lenses (Pentax and Sigma) have this issue.

Thanks,

Adam

12-23-2008, 01:44 PM   #2
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This is most likely due to your subject being dark and low contrast occupying most of the frame. Underexpose the image by 1 stop and it should improve a little. In the end, it comes down to clever choice of composition.
12-23-2008, 01:49 PM   #3
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It's not a subject-matter/lighting issue. It happens in every situation, regardless. I can take a picture of a flat wall and it will be over-exposed compared to another lens taking the exact same pic 30 seconds later.
12-23-2008, 04:14 PM   #4
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That does sound weird. I haven't had this problem so far.

Were you able to do a test on a tripod on the same scene and see how the camera metered on the same aperture for both lenses? If the shutter speeds are very different then you might want to ask for a replacement and see how it'll behave.

12-23-2008, 04:34 PM   #5
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There might be small variations in exposure between lenses and I don't know if it is exposure related or not but I'd first check if the aperture coupling pin was bent or damaged. That could cause the aperture blades to open up a little more than normal, causing the consistent overexposure.

I had an old Sigma manual focus lens which had a similar issue and "fixed" it by re-bending the aperture coupling pin. Tested the result against a calibrated light meter and it solved the overexposure. Not suggesting you do this to your Limited but good to check the aperture coupling pin first.
12-23-2008, 08:21 PM   #6
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Does it happen at all apertures? Could be slow aperture blades - not closing all the way to the selected f-stop in time. Could you post sample pictures?
12-23-2008, 11:11 PM   #7
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Thanks for the ideas. I don't see anything obviously physically wrong with the lens. I'll have to search out some example pics, but it will probably have to wait til after the holiday...
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