Aspect ratio specifies the ratio of width to height. Video frames have a frame aspect ratio, and the pixels that make up the frame have a pixel aspect ratio. Some video cameras can record various frame aspect ratios, and different video standards use different pixel aspect ratios.

Frame aspect ratio describes the ratio of width to height in the dimensions of an image. For example, DV NTSC has a frame aspect ratio of 4:3 (or 4 width by 3 height) and a typical widescreen frame has a frame aspect ratio of 16:9. Many video cameras that have a widescreen mode can record using the 16:9 aspect ratio. Many films have been shot using even wider aspect ratios.

4:3 frame aspect ratio (left), and wider 16:9 frame aspect ratio (right)

Pixel aspect ratio describes the ratio of width to height of a single pixel in a frame. Pixel aspect ratios vary because different video systems make various assumptions about the number of pixels required to fill a frame. For example, many computer video standards define a 4:3 aspect ratio frame as 640 pixels wide by 480 pixels high, which results in square pixels. The computer video pixels in this example have a pixel aspect ratio of 1:1 (square), whereas the DV NTSC pixels have a pixel aspect ratio of 0.9 (nonsquare). DV pixels, which are always rectangular, are vertically oriented in systems producing NTSC video and horizontally oriented in systems producing PAL video.

If you display rectangular pixels on a square-pixel monitor without alteration, images appear distorted; for example, circles distort into ovals. However, when displayed on a broadcast monitor, the images appear correctly proportioned because broadcast monitors use rectangular pixels.

Pixel and frame aspect ratios

A.

4:3 square-pixel image displayed on 4:3 square-pixel (computer) monitor

B.

4:3 square-pixel image interpreted correctly for display on 4:3 non-square pixel (TV) monitor

C.

4:3 square-pixel image interpreted incorrectly for display on 4:3 non-square pixel (TV) monitor