Originally posted by enoeske I've only got a K20d so I don't know much about the video stuff. How exactly does Shutter speed impact video? I assumed the shutter just stayed open so that you could take video with the sensor.
The mechanical shutter stays open, but you use what's called an electronic shutter instead, achieved using the image sensor itself. On-chip circuitry allows it to finalize the exposure as if the shutter had closed, and the image can then be read off.
You can consider each frame of the video to be a still image, and so each frame has its own exposure level -- and by adjusting the shutter speed and gain, you can affect that exposure level without changing the aperture.
Shutter speed has one important impact for video, though. In the same way as with a still image, a fast shutter speed freezes motion, and a slow one blurs it. Thing is, with a video, you've got a fixed frame rate -- so if you have moving subjects and you freeze their motion with a high shutter speed, then there are noticeable "steps" in the position of the subject between frames, without blurring that tells your eye the subject is moving. The result is that the final video can look choppy / stuttery, so it's generally desirable to use a slower shutter speed. (If you're wanting to extract still images from the video frames, then it can conversely be desirable to have a high shutter speed and reduced motion blur.)