Curious 50D 'problem'?
I've just shot two weddings with the 50D in the past week (will post some pics soon). I came across some curious behaviour with my 70-200 2.8IS (and also my 24-70L). Not sure if it's a problem, so thought I'd just throw it in here to see if anyone else has experienced the same.
When focused to infinity and wanting to quickly focus down to a subject closer to the camera, the 50D seems to 'freeze'. In other words, it simply won't adjust focus. Pressing the shutter button has no effect, the lens doesn't even hunt. I've set the 70-200 to close focus at 1.4m, and the subject is further than that, but makes no difference.
The only solution is to manual focus closer to the target, and then the shutter button comes to life and re-engages, focusing quickly. The fact that it's happened once with the 24-70 as well makes me think it could be an issue with the camera, or the contacts.
As an aside, both lenses focused perfectly with the 40D for nearly a year, except the 24-70 was slightly back-focused. On the 50D the 24-70 is dead-on, but the 70-200 seems to be front focusing (especially at 2.8), and sometimes misses focus. I'm yet to determine if this is user error or a problem with the AF.
Just reporting what I'm finding so far. In every other respect the 50D is a big step up from the 40D, especially in terms of resolution. And being able to focus check accurately has saved me many re-shoots this week!