Can anyone please explain the required resolution for stock video. The sites are all over the map in their instructions and requirements. (I'm confused in other words) One even lists 640x480 which I find difficult to believe.

Do they take the good quality DV or HD and offer it in different formats and resolutions?

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I only shoot for iStock now and they have a pretty easy format. You need QT in .mov format. The smallest they take is DV-720x480, In HD they take 1080i use 95% quality on export and you should be fine. If submitting to other sites I would still use this criteria and submit the same to them as well. I shoot mostly HD footage now.