I've been eyeballing getting the Canon 50mm L lens, the f/1.2 one.
Reading the reviews on Amazon.com...I seen references about the lens having a problem with backfocusing.
Can someone explain this to me...what it is, why it happens?
Have others experienced this with this lens? Is it a bug through all of these lenses, or does it happen here and there?
Is this a reason to avoid this lens? If so..why hasn't Canon fixed this?
Thank you in advance,
neuroanatomist below - as always - has given you a great technical explanation already. Let me ad my personal experience: I always loved 50mm lenses already back in the film days. The 50 1.4 has been a reference lens for decades and rightly so. In the EF/EOS world, unfortunately, this great lens has been reduced to a consumer grade plastic lens with a stupid AF clutch design that needs to treated like a raw egg. That was the main reason for me to upgrade to the 50L. And by upgrade I first and foremost mean the build quality which to me personal is very important even though I treat this one like a raw egg as well. Go figure.
It is also an upgrade optically for my specific interests. It is much sharper wide open and the contrast is very special. It's a different lens and a different design. All this comes at a price (and I don't mean just the extra cost). My 1.4 in fact was sharper when shooting in the 2.0 or 2.8 range. Not just for brick-wall test shots but also in real life. So I kept it around for a while. I never used it while the 50L rarely is taken off my 5DII. So I sold the 1.4 even though it was a really good copy without any issues.
So despite this little difference when comparing it A to B the 50L is still the clear winner for me and I wouldn't want it changed in any way. It'll probably happen one day. Canon will come up with some (even more) plasticky new 1.4 design with IS and super sharp and everything. Doesn't interest me. I shoot wide open a lot and even stopped down to 1.6, 1.8 or 2.2 or whatever it still delivers stunning results. I've never noticed the back focus issue even though I now that by design it must be there. I've never even played around with the AF compensation settings. It's fine as it is. Just because of the narrow DOF there will be misses. So what? There will always be stunning hits also.
All the noise on various forums about how the 50L is junk because of the terrible back focus and the CA "issues", all the stupid comparisons of different manufacturing years, all the talk how the 5dII is horrid because of the AF and "banding". It's mostly irrelevant in real life. There is always something and you usually can make it work if you know what you want.
Make sure you know why you want this and then go try one out first. It's too expensive to just have it for the sake of having another hunk of glass with a red ring around it. If sharpness is your prime concern and you stop down usually a bit more the 50 1.4 may be a more reasonable choice. Or other manufacturers for that matter. Or other focal lengths such as the 35L or the 135L. "Sharper" but very different use.