I'm about to upgrade my DSLR camera from a Canon XSi (450D) to either a Canon 7D or T3i (600D). I'm largely interested in the video features of the camera. I have several mid/high-end lenses that I already use for still photography, but I don't have any "general purpose" zoom lenses (since I prefer to use a prime lens for most of my stills).
So, I'd like to get a general-purpose zoom lens (something that covers at least the 20-70mm range, or so) to use for shooting video.
My primary question is, how important is lens quality for videography? I know that "quality" can mean many different things, so let me break this question down a little bit more.
How important is sharpness? 1080p video is barely 2 megapixels, so it would stand to reason that an ultra-sharp lens is not really necessary to get sharp-looking video. Is this an accurate assumption?
My approach for buying lenses for still photography has usually been to buy the best lens I can afford for the application I'm interested in. I'm wondering/hoping that it might be reasonable to buy a cheaper lens for video, and not feel like I'm sacrificing image quality, since the output resolution is so much lower than with stills. Is this just wishful thinking?
Obviously a large aperture is a bonus for video, since long exposures can't be used. But when shooting in low-light, again, it would stand to reason that I might be able to use a higher ISO to compensate for a slower lens, with less noticeable impact, since I'm recording at a much lower resolution than I would be for still photography. Is this also true? How fast of a lens do I need to be able to record in an average indoor lighting setting (no studio lighting), without a high ISO becoming noticeable?
Is image stabilization a pro or a con for hand-held video recording? I can imagine the sound of the motor might be a nuisance if picked up by the microphone. What impact does it have on the video itself?
I'm guessing a USM for auto focus probably won't matter a whole lot, since what I've read suggests that live auto-focus doesn't work very well in video mode. But even so, I suppose a USM AF would be a bonus, if/when it does work.
The ability for a lens to retain focus when zooming seems like it might be of particular importance when shooting video on a zoom lens.
Are there other factors I ought to consider when selecting a lens specifically for video?
I'm not really interested in pointers on which focal lengths I ought to use for what type of shooting, or specific lens recommendations... I'm interested in more general pointers, that will apply to all video shooting.