I was wondering if anyone else who owns this lens has noticed this. First of all, I really like the lens in terms of handling and sharpness. It's superb. I sold my Canon 50/1.2, because I had to stop down to 5.6 to get decent sharpness across the field. Sort of defeats the purpose of a fast lens. But that's another story (and post elsewhere here).
I've owned the lens for about 9 months. On Av, I have no issues with the lens -- it works well. BUT, on manual (which comprises 98% of all my commercial work -- on location and in studio), I've discovered that my images are about 1/3 to even 1/2 stop darker than any of my Canon lenses. So, if my exposure is, let's say, f4 at 1/250, I have to open the lens or the shutter speed another 1/3 or so. Granted, not a huge issue, but bothersome and one that I have to consciously remember to adjust. At first, I didn't realize that this was happening and chalked it up to lighting changing subtly or my screw up.
I sent the lens to Sigma. It checks out fine on their equipment. They don't (needless to say) have access to Canon lens to check what I'm saying, though they accept, without an explanation, that that may be happening. I hate to get rid of the lens, but may do that depending upon what Canon releases.
Anyone else run into this? When I get the lens back, I may post some comparative screen shots.