Rogers comment about dropping a lens certainly aligns with my views. Its why buying a used lens with a unknown history can lead to disappointing results, people often sell a lens because they are just not happy with it even though mechanically its fine. It could have been bumped too hard in shipping or dropped. I've pointed this out to people posting about dropping their lens countless times, but most think that if seems to work fine, how could it be damaged?
I also noticed that the Canon 100mm L tended to have more variation. Mine has the largest AFMA of all my L lenses, and I suspect that it is not totally as good as it should be, but I still like it. I've thought about replacing it, but with my new R, the focus is right on. I once mounted it on a Kenko TC to see if it worked. The AF motor went wild, moving focus in and out slightly like a vibrator, very fast!. After that, it made a noise when it focused. I checked, and the front lens element was loose because the ring that holds it in place had backed off part of a turn. I have the tool to tighten the ring and did, but I've always wondered if the element rotated and is decentered or out of spec. There is nothing obvious though.
"Probably the most pertinent point for you guys is that lenses don’t change over time very much unless something happens. They don’t change much with use, but they can change with abuse, even accidental abuse. If a lens gets dropped (even if it seems fine afterward) or repaired, it may well change optically."
