Is My 24-70 Losing Focus

Professionally, I mostly shoot weddings. My most often used lens is a 24-70 which I will mount onto either my 5D2 or 50D. Normally, it's an excellent performer, but over the past month or two, I've noticed that it has back focused considerably (I may not be using that term correctly. By it, I mean it's focusing past the subject onto something in the background).

I always shoot in One Shot, the lens has been calibrated to both bodies, and my standard approach is to focus with the center spot and recompose. Receptions are dark, of course, and I'm sure the 580's focus assist is coming into play to help.

So I'm wondering, is it me and I'm maybe just being a little sloppy about locking on focus? Or trying to compose and shoot too quickly? Or, despite being in one shot, the lens is refocusing after I recompose? Is the focus assist beam maybe fooling the focusing mechanism after I recompose? Has anyone else experienced this? It doesn't seem to happen to me with other lenses. That said, 80+% of all my receptions shots are through the 24-70. It also doesn't seem to happen with this lens during outdoor shots.

Any insights will be appreciated.

I'll bet most of your backfocused shots are at the wider end of the range rather than the long end. I've been through many copies of this lens and all did the same thing.

It's pretty rare to find any zoom that has dead-on focus throughout it's range .. my 24-70 is about 5 micro adjust off at either end and dead perfect at 50mm.

So maybe you're shooting more lately at one end or another than you normally would so you're now noticing it?

Shooting a wedding last night, I had similar results to the extent I just manually focused the shots during the ceremony. On the wide end about 60 feet from the bride and groom using center point only with single shot AF, it would consistently focus a few pews back even though those were on the margins of my framing and the bride and groom were well lit and taking up a good portion of the center of my frame. Going to send my bodies and lenses in for calibration I think.

I've experienced this problem, but it's been very slight, usually not enough to ruin a shot. I've been meaning to test the focus and micro adjust, but can it be sent for calibration and fixed? I seem to recall reading that it was a problem in the design of the lens.

I just had mine calibrated, but at the wide end focus is inconsistent where focus is off by a hair. What's the point of such an expensive lens when you can't use one part of the focal length? This lens really bugs me! I'll send it in again for round two.