Canon Rebel T2i Quick Start Manual page 5

Video vs Still Image
• Since this is a still camera with a video option, you may be tempted to
record video with the camera in a vertical position. Never do this
(unless you have a special vertical video installation in mind). Keep
the camera horizontal. The video image is always wider than taller.
• Don't expect the video resolution to be as fine as still images. HD
video has a resolution of 1920 x 1080 pixels. This is like shooting with
the small JPEG still image option on the camera.
• Although it is a 18 megapixel camera, video recording does not use all
of these pixels on the sensor. The video image will be noisy in low
light (high ISO) situations.