Why do I still see horizontal bars when watching a "widescreen" movie on my widescreen TV?

Widescreen television receivers are now the norm for new TV purchases. The advent of high-definition television (HDTV) and the move away from old-fashioned analog television (and cable) means that more and more people are upgrading their TVs in search of a more movie-like experience.

So why do you still see horizontal bars at the top and bottom of your TV when you watch a "widescreen" movie?

TV Aspect Ratios

The key lies in the aspect ratios of the TV and the movie. An aspect ratio is the ratio between the length of an individual frame and its height. In standard broadcast TV, the aspect ratio is 4:3, which means that there are 4 units length (horizontal) for every 3 units of height (vertical). The 4:3 ratio can also be expressed as 1.33:1 if you limit the significant digits.

Widescreen TV, on the other hand, has a 16:9 aspect ratio. This means that there are 16 units of length for every 9 units of height. Another way of saying it is that there are 5.33 (5 and a third) units of length for every 3 units of height. This is why widescreen TVs are wider than broadcast TVs — there are more horizontal units for each unit of height. The 16:9 ratio can also be expressed as 1.78:1, again limiting the significant digits and rounding up.

Movie Aspect Ratios

Here's where things break down, though. Movies are filmed in a wide variety of aspect ratios. According to What Is Widescreen?, those ratios today vary from 1.85:1 to 2.4:1, though other ratios are possible.

What this means is that if you display a movie such that the movie takes up the full height of the TV screen, some of the width gets chopped off. The only way to combat this is to shrink the movie vertically until its width is exactly the size of the screen. Thus you end up with black bands at the top and bottom of the screen, and the height of those bands will vary depending on the aspect ratio of the movie.