What factors are important for DSLR lenses for shooting video?
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?

It depends on the way the DSLR is capturing it's video from the sensor. The first method is the most obvious one, take the image and scale it, but there's also another; use only the pixels across the sensor's area that approximately &quot;line up&quot; with the pixels in the final resolution. I.e. in a 1080p image only every third or fourth scan line would be used resulting in one sensor pixel being used to produce one pixel in the video image.
I would hazard a guess that an &quot;acceptably sharp&quot; lens would be okay for video, though not ideal and probably for more reasons than image sharpness.

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?

As my above answer suggested, there are other aspects of the lens you need to keep in mind when choosing a video lens and these may be:

Maximum aperture for low-light.
Fixed aperture for consistent exposure when zooming.
Image stabilisation is especially useful when not using a steadicam or other stabilising rig or device.
Ease of use, especially when it comes to the focus ring. Canon USM lenses in particular have a very smooth, usually well placed and grippy focus ring. Kit lenses aren't so good in comparison.


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?

I think you'll find with most typical video DSLR's is that the ISO performance for video isn't quite as great as you might assume, but it in no way bad. I'd say ISO 1600-3200 lives up to most expectations on the 18mp APS-C sensors, but beyond that it's quite 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?

For music videos and other videos where sound is not used, IS is fantastic and goes a long way to ensuring the video is jitter free, but if you need the sound and it's potentially a quiet environment, it might be prudent to use an external, higher quality mic attached to say, the hot shoe via a mount.

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.

As above, Canon USM lenses (whether L or standard) have very usable focus rings. Provide good grip and are very smooth to focus with.

I personally use a Canon EOS 60D with an EF-S 17-55mm F/2.8 IS USM lens and find it a great combination at those focal lengths. The IS in that lens is quite remarkable and the USM focus ring makes it very smooth and accurate to focus. When I turn the IS off, I can instantly see the jittery movement from my hands trying to hold a 2kg camera setup.