I've been noticing lately that when pulling focus on my Sigma 24-70 lens with my 60D, that elements of the image distort a little bit as I focus. It's hard to explain, but I guess it seems to happen around the edges sort of like barrel distortion. Should I be expecting this from this lens? Is it simply a matter of getting a more expensive lens more suited for video, or is it just the way that type of lens is designed?

I'll see if I can post an example later.

I have never use that lens, but if it's still under warranty I would talk to Sigma and send it in to them. I do see where people have said it does create a bit of vignette.

Yeah, the lenses does breath, or at least mine does, but I don't think it's been noticeable in any of my work.

That's going to be the issue with using photography lenses for video.