An aspect ratio is the ratio between the width and height of a film image. The number denoting width comes first, and the height portion of the ratio is always written as 1. A motion picture's aspect ratio often appears on the back of the DVD or video box. An example would be 1.85:1. This means that the size of the original theatrical presentation of that film is 1.85 times as wide as it is high.
Prior to the early 1950s, almost all motion pictures had the aspect ratio 1.33:1. This ratio was recognized by the Academy of Motion Picture Arts and became known as Academy Standard. When television standards were being developed in 1941, the National Television Standards Committee, or NTSC, decided that 1.33.1 would be the aspect ratio for television sets and broadcasting in the United States. This ratio is also written as 4x3 and is used on all non-widescreen television sets. Technically, the Academy Standard ratio is really 1.37:1, but it is still commonly referred to as 1.33:1.
To compete with the decline in theater attendance resulting from television sales, the film industry began experimenting with different formats. The widescreen aspect ratio was a result of this experimentation, and was first used in the 1950s. Widescreen is achieved by taking a rectangular image with the camera lens while filming, and compressing it horizontally to fit on the square film negative. When the film is projected in theaters, a magnifying lens on the projector restores the original aspect ratio. Many different ratios were developed, but the two most common are 1.85:1 and 2.35:1.
1.85:1, also known as Academy Flat, is more rectangular than Academy Standard. 2.35:1 is even wider and is called CinemaScope, Anamorphic Scope, or Scope. This aspect ratio actually changed to 2.39:1 in the 1970s when the CinemaScope process was replaced by Panavision and Anamorphic Scope, but these are all still commonly referred to as 2.35:1.
Many other aspect ratios are used in moviemaking, such as the European standard of 1.66:1, as well as some that have only been used for a few films, such as 2.76:1. To maintain a widescreen look on 4x3 televisions, the full rectangular image is shown in the middle of the screen and black bars fill the unused top and bottom — a process called letterboxing. Full-screen versions can be made with the original aspect ratio compromised by shooting only the part of the image that fits on a 4x3 set, a technique called "pan and scan." These videos contain a message explaining that the film has been modified to fit the screen. Widescreen televisions, which have an aspect ratio of 16x9, can show most widescreen films on the entire screen.