The aspect ratio of an image is the ratio of its width to its height. Beginning in the 1910s, motion pictures settled on a ratio of four to three (four units wide to three units high). Often written as 4:3, this ratio may be reduced to 1.33:1 and this aspect ratio is commonly known as 1.33. The introduction of sound-on-film narrowed the aspect ratio briefly, before the Academy ratio of 1.37 was introduced in 1932 by means of thickening the frame line. For years, cinematographers were limited to this shape of image, but in the 1950s, thanks to the unanticipated popularity of Cinerama, widescreen ratios were introduced in an effort to pull audiences back into the theater and away from their home television sets. These new widescreen aspect ratios granted cinematographers a wider frame within which to compose their images. Many different proprietary photographic systems were invented and utilized in the 1950s to create widescreen movies, but one dominates today: the anamorphic process, which optically squeezes the image to photograph twice the horizontal area to the same size vertical as standard "spherical" lenses. The first commonly used anamorphic widescreen format was CinemaScope, which used a 2.35:1 aspect ratio, although it was originally 2.55:1. CinemaScope was used from 1953 to 1967, but due to technical flaws in the design and its ownership by Fox, several third-party companies, led by Panavision's technical improvements in the 1950s, now dominate the anamorphic cine lens market. Changes to SMPTE projection standards altered the projected ratio from 2.35:1 to 2.39:1 in 1970, although this did not change anything regarding the photographic anamorphic standards; all changes in respect to the aspect ratio of anamorphic 35 mm photography are specifically correlative to camera or projector gate sizes, not the optical system.

After the "widescreen wars" of the 1950s, the motion-picture industry settled into 1.85:1 (which is a cropped version of 1.37:1) as a standard for theatrical projection in the United States and the United Kingdom. Europe and Asia opted for 1.66:1 at first, although 1.85:1 has largely permeated these markets in recent decades. Certain "epic" or adventure movies utilized the anamorphic 2.39:1.

In the 1990s, with the advent of high-definition video, television engineers created the 1.78:1 (16:9) ratio as a mathematical compromise between the theatrical standard of 1.85:1 and television's 1.33:1, as it was not physically possible to safely create a television tube with a width of 1.85:1. Until that point, nothing had ever been originated in 1.78:1. Today, this is a standard for high-definition video and for widescreen television.