The aspect ratio of an image is the ratio of its width to its height. It is commonly expressed as two numbers separated by a colon, as in 16:9. For an x:y aspect ratio, the image is x units wide and y units high. Widely used aspect ratios include 1.85:1 and 2.39:1 in film photography, 4:3 and 16:9 in television, and 3:2 in still camera photography.
The most common aspect ratios used today in the presentation of films in cinemas are 1.85:1 and 2.39:1. Two common video aspect ratios are 4:3 (1.3:1), the universal video format of the 20th century, and 16:9 (1.7:1), universal for high-definition television and European digital television. Other cinema and video aspect ratios exist but are used infrequently.
In still camera photography, the most common aspect ratios are 4:3, 3:2, and more recently found in consumer cameras, 16:9. Other aspect ratios, such as 5:3, 5:4, and 1:1 (square format), are used in photography as well, particularly in medium format and large format.
With television, DVD and Blu-ray Disc, converting formats of unequal ratios is achieved by enlarging the original image to fill the receiving format's display area and cutting off any excess picture information (zooming and cropping), by adding horizontal mattes (letterboxing) or vertical mattes (pillarboxing) to retain the original format's aspect ratio, by stretching (hence distorting) the image to fill the receiving format's ratio, or by scaling by different factors in both directions, possibly scaling by a different factor in the center and at the edges (as in Wide Zoom mode).
In motion picture formats, the physical size of the film area between the sprocket perforations determines the image's size. The universal standard (established by William Dickson and Thomas Edison in 1892) is a frame that is four perforations high. The film itself is 35 mm wide (1.38 in), but the area between the perforations is 24.89 mm × 18.67 mm (0.980 in × 0.735 in), leaving the de facto ratio of 4:3, or 1.3:1.
With a space designated for the standard optical soundtrack, and the frame size reduced to maintain an image that is wider than tall, this resulted in the Academy aperture of 22 mm × 16 mm (0.866 in × 0.630 in) or 1.375:1 aspect ratio.
The motion picture industry convention assigns a value of 1.0 to the image's height; an anamorphic frame (since 1970, 2.39:1) is often incorrectly described (rounded) as 2.40:1 or 2.40 ("two-four-oh"). After 1952, a number of aspect ratios were experimented with for anamorphic productions, including 2.66:1 and 2.55:1. An SMPTE specification for anamorphic projection from 1957 (PH22.106-1957) finally standardized the aperture to 2.35:1. An update in 1970 (PH22.106-1971) changed the aspect ratio to 2.39:1 in order to make splices less noticeable. This aspect ratio of 2.39:1 was confirmed by the most recent revision from August 1993 (SMPTE 195-1993).
In American cinemas, the common projection ratios are 1.85:1 and 2.39:1. Some European countries have 1.6:1 as the widescreen standard. The "Academy ratio" of 1.375:1 was used for all cinema films in the sound era until 1953 (with the release of George Stevens' Shane in 1.6:1). During that time, television, which had a similar aspect ratio of 1.3:1, became a perceived threat to movie studios. Hollywood responded by creating a large number of wide-screen formats: CinemaScope (up to 2.6:1), Todd-AO (2.20:1), and VistaVision (initially 1.50:1, now 1.6:1 to 2.00:1) to name just a few. The "flat" 1.85:1 aspect ratio was introduced in May 1953, and became one of the most common cinema projection standards in the U.S. and elsewhere.
The goal of these various lenses and aspect ratios was to capture as much of the frame as possible, onto as large an area of the film as possible, in order to fully utilize the film being used. Some of the aspect ratios were chosen to utilize smaller film sizes in order to save film costs while other aspect ratios were chosen to use larger film sizes in order to produce a wider higher resolution image. In either case, the image was squeezed horizontally to fit the film's frame size and avoid any unused film area.
The development of various film camera systems must ultimately cater to the placement of the frame in relation to the lateral constraints of the perforations and the optical soundtrack area. One clever wide screen alternative, VistaVision, used standard 35 mm film running sideways through the camera gate, so that the sprocket holes were above and below the frame, allowing a larger horizontal negative size per frame as only the vertical size was now restricted by the perforations. There were even a limited number of projectors constructed to also run the print-film horizontally. Generally, however, the 1.50:1 ratio of the initial VistaVision image was optically converted to a vertical print (on standard four-perforation 35 mm film) to show with the standard projectors available at theaters, and was then masked in the projector to the US standard of 1.85:1. The format was briefly revived by Lucasfilm in the late 1970s for special effects work that required a larger negative size (due to image degradation from the optical printing steps necessary to make multi-layer composites). It went into obsolescence largely due to better cameras, lenses, and film stocks available to standard four-perforation formats, in addition to increased lab costs of making prints in comparison to more standard vertical processes. (The horizontal process was also adapted to 70 mm film by IMAX, which was first shown at the Osaka '70 Worlds Fair.)
Super 16 mm film was frequently used for television production due to its lower cost, lack of need for soundtrack space on the film itself (as it is not projected but rather transferred to video), and aspect ratio similar to 16:9 (the native ratio of Super 16 mm is 15:9). It also can be blown up to 35 mm for theatrical release and therefore is sometimes used for feature films.