My theory about this is that every lens and body need a little bit of calibration to work together optimally. This isn't because they are broken or otherwise defective, it's just that they need the adjustment to focus as accurately as possible. The latest Canon bodies (the 1D3, 1DS3, 50D, 5D2) provide user control to make this adjustment. Without this calibration, a lens that tests sharp on one body will seem soft on another. Same lens, different result! I recently upgraded from a 30D to the 5D2. One reason for choosing the 5D2 over the less expensive 5D (which would have been satisfactory in many ways) was the ability to make this adjustment myself.


The irony of this is that fast lenses, which tend to be the most expensive, are also the ones that most likely need adjustment. This is because their depth of field is so shallow that even the slightest miscalibration will show up. Lenses intended for consumers (cheaper kit zooms and tele zooms) have enough depth of field that you can't tell if they're off.


I did some initial calibration tests with my 5D2 and found that the f/4 lenses were calibrated well enough that without spending a whole lot more time testing I didn't think I could confidently improve focusing accuracy. An 85/1.2 lens seemed to improve with a bit of adjustment, but there was enough inconsistency in focus from test shot to test shot that I couldn't tell if it was the adjustment or the inconsistency that I was seeing. The lens that needed it the most was the 50/1.4, which was significantly off and much improved with adjustment. Hooray, it worked!


I don't have any f/2.8 zooms, but here I think adjustment would make a difference. A lot of people have complained about the 24-70/2.8 zoom being soft out of the box and going through several returns to get a good one. I'm pretty certain that the problem was that the calibration was off and that any of them would have been fine. It's always possible to get a lemon, but to get three in a row is really unlikely, and I have lost enough money at slots to be very confident in saying so.


In a final note, I think that most people expect "pro" equipment to be more reliable and better adjusted out of the box than "consumer" equipment. From a manufacturer's perspective, the opposite is true! If you ship 1000 units at a 1% defect rate to pros who have local pro service centers, it's entirely managable. If you ship 1,000,000 units to consumers at that same defect rate, you have a nightmare! I'm not saying that Canon neglects testing and manufacturing controls on their pro equipment, but I don't think that it necessarily gets "special" treatment either.