Question about t4i video resolution
It's because the SD resolution isn't 16:9. You either want HD or SD, and they're different aspect ratios. When I reduce the resolution to SD from HD, the video maintains its HD aspect ratio, but you have to go through the editing in HD before exporting to SD.
What you should be doing is saving all of the HD files to your scratch drive, editing, and then exporting a full-resolution file for storage and a desired-resolution file for distribution. Unless your camera manual suggests that you can do otherwise, you're stuck with doing the work yourself, or buying an SD camcorder instead.
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