I love the classic 50mm view with its natural look and feel without either tele-compression or any wide-angle effects. 50mm lenses are small lightweight, fast and (probably) relatively easy to design and manufacture. In fact you can purchase a Canon EF 50mm f/1.8 for just 100USD however the EF 50mm f/1.2, just slightly faster and much better build quality comes with a killer price tag at a staggering 1500USD. At this price level it should be the perfect 50mm without any issues.
It has taken me a long time to understand this lens and now after 3 years I am somewhat at peace with it. In spite of much frustration and low keeper percentage it is slowly becoming one of my favorites because when it delivers the results are wonderful. In focus isolation, bokeh and the way the 50 renders out of focus points of lights are distinctly different to the characteristics of for example the 85 & 135. The out of focus droplets at medium photo distances are quite small making the background visible. On some backgrounds the effect can be a bit “disturbing” but often the visibility of the background (yet out of focus) makes the background included in the context of the image (as compared to the more total out of focus blur characteristics of for example the 135/2).
The mentioned low keeper rate are all related to the difficulty for the fifty to really nail critical focus. But when focus is 100% the results are great. Wide open it is soft and many optical aberrations become painfully visible and are exaggerated even if only slightly out of focus. In most portrait situations I prefer to use this lens from F1.8 and from F2.8 it starts to get really good, but that is not what you expect when you invest 1500 USD.
Many have written about focus issues with this lens. My own frustration has made me do many different tests but to make a long story short and contrary to many others opinion I find that in good high contrast test-conditions the 50 is both accurate and consistent, with no real issues. But in reality the problems starts as soon as target contrasts are less than perfect. Of course focus point accuracy drops as contrast goes down and sooner or later all lenses will suffer and give inconsistent results, but for some reason the 50 is more sensitive to this effect. It will fail in situations where the others, for example the 85, 135 are still accurate.
On the positive side once you recognize the tricky situations it is possible to be careful and work around the problem, at least to some extent. The best cure is simply to ensure enough contrast at the focus point … this issue applies to day-light backlit situations as well as in general low light situations. In dark (night) portrait situations a single candle reflecting light on the eyes can be the cure and in backlit daytime photo a reflector bouncing light into the model face is often the best cure also for AF accuracy. Other work-a round’s are LIVE VIEW focusing and also EG-S focus screen can help a bit. But moving targets in low contrast light is a hopeless case for this lens at least together with 5D2:
More on this subject can be found on Canon’s own homepage from their master photographers stories … apparently one of the main reasons to upgrade to 1Dmk4 (from 5Dmk2) is to get focus system you can rely on – “I've never really used AI Servo as I've never trusted it” (comment by Jeff Ascough wedding photographer on 5d2 performance …. 50/1.2 & 24/1.4 are his main tools)