Help diagnose what I think is a faulty lens?

I posted earlier in the year about concerns with a Canon 24-105 lens that I purchased (soft focus concerns). I wasn't using it much at the time for night photography, but have since used it at night quite a bit. The lens seems to consistently overexpose (day and night), and it has been producing the effect below, where the bottom 1/2 of the image looks like I think it should (a night shot), and the top half looks like it was taken during the day.

Both shots are taken with a 50D, from a tripod, shutter release, in AV mode, extended exposures. The first shot was taken at 6:45pm (official sunset was 4:55pm). It was dark by this point; not pitch black, but definitely not this much light in the sky. The second image, again, was taken probably 2 hours after sunset.

This "effect" only happens with this lens. Doesn't happen with my Canon 10-22 or 18-135 kit taking the same image seconds before or after using this lens.

I've sent the lens to Canon for inspection, but in the interim, I'm wondering if anyone has any ideas about why this is happening. I appreciate any input.

ISO 100; f5.6; 30 seconds

ISO 100; F8; 8 seconds

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Replies to This Discussion

Permalink Reply by CameraClicker on January 8, 2012 at 6:14am

If other lenses provide expected results and this lens is consistently over exposing, my conclusion would be the aperture is not working/calibrated properly. Presumably if you shot wide open, you would get the exposure you are expecting.

Permalink Reply by DJK on January 8, 2012 at 8:46pm

Thanks, CameraClicker. That's what I suspect. I hope to hear back from Canon this week.

Permalink Reply by Janna Slaback on January 21, 2012 at 8:23pm

I know you are not completely pleased with the photos but the second one is so beautiful, in my opinion!!

Permalink Reply by DJK on January 22, 2012 at 9:20pm

Thanks, Janna! I have several similar photos on my page that are much better than the second one.

Permalink Reply by DJK on January 22, 2012 at 9:39pm

For those interested, the lens is back from Canon. Apparently the autofocus assembly wasn't working correctly, causing inaccurate auto focus. There was no mention at all about the overexposure issue (the reason I sent the lens in in the first place) in the service repair log or in Canon's system. Disappointing, but I was assured if there was a problem, they fixed it.

Permalink Reply by CameraClicker on January 23, 2012 at 4:49am

Some strange things happen in repair shops. Test it. Someone may have selected the wrong repair code.

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