HDTVs are made to accommodate standard definition (SD) programs, which will exist for decades in the form of VHS tapes, DVDs and thousands of movies in the cable, satellite and network TV archives. Filling a 16:9 wide screen with the more square 4:3 source material is done by zooming or stretching. There is also a setting that maintains the original aspect ratio by displaying black or gray bars on either side (see below). See aspect ratio
Zooming (No Distortion - Content Removed)
The 4:3 frame is cropped at the top and bottom, and the remaining image is scaled up to fit the wide screen. Although people's heads are sometimes cut off in this mode, if objects mostly appear in the center of the frame, zooming is effective because there is no distortion of the original image. For letterbox movies, this is often the best setting (see below).
Stretching (Distorts Image - No Content Lost)
The 4:3 frame is stretched horizontally to fit the 16:9 screen. Stretching is done either uniformly across the screen or only at the sides, with both options available on some sets. Since only the width is scaled and not the height, the image is distorted. Some sets have a combo mode that stretches and zooms.
All HDTVs render these modes with a slightly different twist, and any purchasing evaluation must be made on the set in question, not another brand or model. See HDTV
Standard TV on HDTV Sets
Even when everyone has a wide screen TV, a huge amount of video material will still be available in the standard format.
HD on HDTV Sets
The wide screen on an HDTV accommodates movies in their original formats. When a movie is in a format wider than the 16:9 HDTV aspect ratio, the image can be zoomed to eliminate the letterboxes.