Aspect Ratio describes the visual size of an entertainment medium such as television or film in terms of the image's width compared to its height.

For example, most older television sets in the United States have a ratio of 4:3 (1.33:1, known as "Academy Ratio"), meaning that the image you see is four units wide and three units tall. Widescreen televisions use 16:9 (1.78:1), but this format didn't really catch on until UsefulNotes/HighDefinition television broadcasts caught on. Interestingly, most (but not all) UsefulNotes/HighDefinition cameras used for film production shoot at a 16:9 ratio.

The most common aspect ratios for feature films today are 1.85:1 (generally used for comedy, drama, or other small scale projects without much emphasis on set design or scenery) or 2.35:1 (generally action, science fiction, "epic scope", and other large scale projects). Most older films used the 4:3 aspect ratio; the "Academy" in Academy Ratio is the same one as in the UsefulNotes/{{Academy Award}}s, and they set the standard ratio back in 1932. It was the uprising of television in the fifties that led to the film industry seeking out methods of innovation to stay competetive. A lot of attempts at innovation were gimmicky and didn't last long - the first round of 3D, for instance - but two, the increased use of Color film and wider aspect ratios, proved enduring. Various ratios were tried; the widest ratio of all was Cinerama, which used three projectors to display a 2.89:1 image[[note]] problems with the synchronisation of the three projectors doomed Cinerama as a serious production format, but in terms of sheer spectacle, it was a precursor to IMAX.[[/note]]. One should keep in mind that wider aspect ratio is not necessarily larger; proper IMAX is the largest screen in the world, but only has a ratio of 1.44:1 (13:9).

Because the WidescreenShot of a feature film is so often different from that of a television screen, a movie image will not fit squarely within a TV screen when released for home video or shown on broadcast television. In order to get around this problem, the aspect ratio of the film must be altered. There are a few common methods for doing this: PanAndScan, {{Letterbox}}, and VisualCompression.

As people still hold onto earlier media as TechnologyMarchesOn, some have noticed that older [=DVDs=] that advertise themselves as {{Letterbox}} are actually designed for 4:3 screens and will not "fill" newer TV sets, despite matching the aspect ratio on a smaller scale. This is because they were released when DVD was taking off in the late 90's when almost every computer monitor and TV screen was 4:3. Some newer televisions have a zoom function for this, though the zoomed image may seem pixelated.

Aspect Ratio can also be an issue with cameraphone footage. CharlieBrooker has complained about people filming with their phones held upright, producing a tall, narrow image instead of the "correct" widescreen ratio[[note]]Not all cameraphones actually shoot in 16:9 widescreen - depending on the camera component, the screen dimensions, and the software, ratios can vary from 1.33:1 all the way up to 1.77:1 and beyond[[/note]]. WebVideo/GloveAndBoots would like you to take three minutes to watch their PSA on the subject [[https://www.youtube.com/watch?v=Bt9zSfinwFA here]]. If you want to learn more about the evolution of the Aspect Ratio in movie history, check out [[https://vimeo.com/68830569 this webcast]].