Several recent threads have talked about inferior copies of Canon L lenses, copies that have had to be returned. Sometimes more than once.

So what may be a dumb question, but assuming that the lens is not actually broken - won't focus properly or has loose elements - how do you know that your copy is inferior? What are you comparing it to?

Funny you started this thread today, as I dropped off my 50L at Canon Irvine for it's infamous back focus problem. I hope they can fix it.

Of the 15 L lenses I own, the 50L is the first that required a return to the Canon service center. I have to say that I'm very pleased with Canon's quality control, at least for the 14 other L's I have. I have a sneaky feeling however my 50L may not be fixable, as I suspect the issues are with the design.

To your question, I would suggest you test the lens yourself, tripod, newsprint on wall, mirror locked up, remote shutter release, and put it through it's paces.

Often "bad" AF performance is attributed to camera workflow. For example, often people will shoot with a fast aperture and close, lock focus, then recompose, and of course this will often result in a soft image.

I've been wondering about this issue myself. Having shot a theater performance last week, I am becoming more and more convinced that I have a "bad' copy of a 24-105. I used two lenses- the 24-105 and the 70-200 2.8. In virtually every shot, the 70-200 has a measurable increase in sharpness. The 24-105 appears to be *slightly* out of focus, or soft, in most, but not all, cases. I would describe it as getting lucky with the focus at times. The 70-200, even wide open, is amazingly sharp across the range of focal lengths.

Because of my suspicions about this lens, I did a series of tests with it at a complete range of focal lengths and apertures with, get this, NO problems. I even did the whole series with the IS on and off. Granted, the tests were all done tripod mounted, not handheld, but if there was a general problem with this copy, would it not be revealed either way?

As a matter of note, I have also preformed the AF Microadjustment on this lens per the instructions on the Northlight Images site. It was off a few clicks, but very minor.

What gives on this lens, and how would I go about having it professionally evaluated?

If your lens works when tripod mounted but not when handheld then I guess that there is either a problem with the IS or you are not holding the camera as steady as you think you are. But since you write that images with the 24-105 are soft but not blurred, that does not make sense either. I do all my work on tripod with the mirror locked up and have noticed the vibration in the tripod legs (GITZO 1325 CF) when the mirror is released. Maybe the mirror slap is causing the for the lighter lens that you think is soft focus?

I own three L lenses (28-70, 70-200 and 400) as well as the non-L macro and find all to be really sharp. But maybe that is because they are always tripod mounted and usually closed down to f8 or 11. I was just surprised to read how may people are having quality issues with the L lenses.

As I noted, the shots taken with the 70-200, wide open, under the same conditions, were tack sharp. As far as L lenses go, I also have a 100-400 that is spot on so I am really at a loss on this 24-105. Perhaps I need to do some more rigorous testing?

Beats me. If you're confident that it isn't mirror slap effecting the 24-105 (which is lighter than either the 70-200 or 100-400) than maybe you should send it to Canon for testing.

How do I go about doing that? I purchased the lens locally; should I start there?

Try searching the web for the Canon service center nearest to where you live. Your local camera store might also be able to send the lens in for service for you.