Image Dimensions and Aspect Ratio
The horizontal and vertical pixel dimensions of your format determine the frame size
and aspect ratio. For example, SD NTSC video is 720 pixels wide and 480 pixels tall.
HD video is either 1280 x 720 or 1920 x 1080, and is usually referred to by the vertical
dimension and the frame rate (for example 720p60 or 1080i30).
The aspect ratio of a video frame is the width with respect to the height. SD video has
an aspect ratio of 4:3, while HD video uses 16:9.
Note: You may notice that 1280/720 or 1920/1080 is equivalent to 16:9, while 720/480 is
not equivalent to 4:3. This is because SD digital video uses pixels that are rectangular,
not square. For more information, see Volume IV, Appendix A, “Video Formats.”