10 out of 10 points and recommendedAn excellent low-light performer, also good for full body portraitsnone, if you know how to use it.
A common problem with any large aperture lens is that the focus actually shifts for the center point when you stop down. Since all cameras auto-focus with the aperture wide open, you in effect get back focusing. In other words, you're not back focusing. It was in focus in the viewfinder, but when the lens stopped down, the focus shifted.reviewed November 1st, 2007 (purchased for $1,200)
1) why are you using the center focus point anyway? The rule of thirds is a guideline for you? Use any other focus point besides the center focus point when you are using a smaller aperture than f1.2.
2) The only reason to use the center focus point is that on some SLR's the center is a cross type and the others are horiz/vert only or they are less sensitive. In that case, open the aperture all the way and you don't have a focus problem. The only reason you need the extra precision of the center focus point is that it's low light anyway. In moderate light, auto-focus with another focus point. In moderate light, you don't need a cross type or super-sensitive focus sensor.
With just those two points, autofocus on an ef-50 1.2 L is cake. In any case, manual focus is also cake if you swap out your focus screen. (If you're using a rebel, 20d or 30d, (can't take out your focusing screen), then I see why you have to insist on autofocus.)
This means: a) there's no defect. It's a law of optics. Complaining about this makes as much sense as complaining about gravity. b) Calibrating won't help unless you shoot exclusively wide open. c) No, you can't program the lens logic to compensate for this effect, because the amount of offset depends on distance to subject, amount you are stopping down by and a few other factors I don't remember since seeing the equations for this. Yes, the lens can guess what the distance to subject is, but that's just precise enough for flash exposure in E-TTL II, but not nearly good enough for focus. What's next, a laser distance finder duct-taped to the lens?
The tanner report that's linked is useless, because he's comparing two different things. Of course the lens will perform poorly. He's taking the worst settings for the 1.2L compared to the worst settings for the 1.8. However, if you set the 1.2L to 1.8 or better yet, f2.0, you get an extremely sharp picture. It also spends so much time on graphs; it ignores what we should be using this lens for: taking pictures. At 1.2L and iso 800, you can almost shoot in the dark without flash or tripod. At wedding receptions, when the lights dim and people start dancing, I can take memorable pictures and the customers don't notice I'm there, because I didn't use a huge flash.
That's why Canon doesn't admit a defect in the lens-there's no defect. It's a basic law of optics that's giving you problems. To make the focus mongers happy, the autofocus would have to work after the lens is stopped down. This would be a problem since these same people would say, "Why's my viewfinder so dark? They did give us a DOF preview button. Use that and manual focus, OR use those two points above and this lens will come alive for you.