Aspect ratios are mathematically expressed as x:y
The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. That is, for an x:y aspect ratio, no matter how big or small the image is, if the width is divided into x units of equal length and the height is measured using this same length unit,
the height will be measured to be y units. For example, consider a group of images, all with an aspect ratio of 16:9. One image is 16 inches wide and 9 inches high. Another image is 16 centimeters wide and 9 centimeters high. A third is 8 yards wide and 4.5 yards high.
Aspect ratios are mathematically expressed as x:y (pronounced "x-to-y") and x×y (pronounced "x-by-y"). The most common aspect ratios used today in the presentation of films in movie theaters are 1.85:1 and 2.39:1. Two common videographic aspect ratios are 4:3 (1.33:1), universal for standard-definition video formats, and 16:9 (1.78:1), universal to 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 being found in consumer cameras 16:9. Other aspect ratios, such as 5:4, 6:7, and 1:1 (square format), are used in photography as well.
With television, DVD and Blu-ray, converting formats of unequal ratios is achieved by either: 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, or (for TV and DVD) by stretching (and sometimes distorting) the image to fill the receiving format's ratio. Cinematographic aspect ratios are usually denoted as a decimal multiple of width vs unit height, whilst videographic aspect ratios are usually defined and denoted by whole number ratios of width to height.
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 1.33:1.
With a space designated for the standard optical soundtrack, and the frame size reduced to maintain an image that is wider than tall (mimicking human eyesight), this resulted in the Academy aperture of 22 mm × 16 mm (0.866 in × 0.630 in) or 1.37:1 aspect ratio.
The motion picture industry convention assigns a value of 1.0 to the image’s height; thus, an anamorphic frame (actually 2.39:1) is described (rounded) as 2.40:1 or 2.40 ("two-four-oh"). In American cinemas, the common projection ratios are 1.85:1 and 2.40:1. Some European countries have 1.66:1 as the wide screen standard. The "Academy ratio" of 1.37:1 was used for all cinema films until 1953 (with the release of George Stevens's Shane). During that time, television, which had a similar aspect ratio of 1.33:1, became a threat to movie audiences, Hollywood gave birth to a large number of wide-screen formats: CinemaScope, Todd-AO, and VistaVision 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.
Movie camera systems
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 frame, allowing a larger horizontal negative size per frame as only the vertical size was now restricted by the perforations. However, the 1.50:1 ratio of the initial VistaVision image was optically converted to a vertical print (on standard 4-perforation 35 mm film) to show in the 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 1970s for special effects work that required 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 4-perforation formats, in addition to increased lab costs of making prints in comparison to more standard vertical processes. (The horizontal process was later adapted to 70 mm film by IMAX.)
Super 16 mm film is 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 1.66:1 while 16:9 is 1.78:1). It also can be blown up to 35 mm for theatrical release and therefore is also used for feature films.
Current video standards
The 4:3 ratio (generally named as "Four-Three", "Four-by-Three", "Four-to-Three", or "Academy Ratio") for standard television has been in use since television's origins and many computer monitors use the same aspect ratio. 4:3 is the aspect ratio defined by the Academy of Motion Picture Arts and Sciences as a standard after the advent of optical sound-on-film. By having TV match this aspect ratio, films previously photographed on film could be satisfactorily viewed on TV in the early days of the medium (i.e. the 1940s and the 1950s). When cinema attendance dropped, Hollywood created widescreen aspect ratios (such as the 1.85:1 ratio mentioned earlier) in order to differentiate the film industry from TV.
16:9 (generally pronounced as "Sixteen-by-Nine"; alternates include "Sixteen-Nine" and "Sixteen-to-Nine") is the international standard format of HDTV, non-HD digital television and analog widescreen television (EDTV) PALplus. Japan's Hi-Vision originally started with a 5:3 ratio but converted when the international standards group introduced a wider ratio of 5⅓ to 3 (=16:9). Many digital video cameras have the capability to record in 16:9. Anamorphic transfers onto DVD horizontally squeeze the original widescreen image to store the information into a 4:3 aspect ratio DVD frame. If the TV has a feature to un-squeeze an anamorphic image, it will horizontally decompress the image to 16:9. If not, many DVD players can also reduce scan lines and add letterboxing bars above and below the image before sending it to the TV. This is made easier by the simple, 4:3 ratio between 4:3 and 16:9 (16:9 = 4:3 × 4:3). DVD producers can also choose to show even wider ratios such as 1.85:1 and 2.39:1 within the 16:9 DVD frame by hard matting or adding black bars within the image itself. Some films which were made in a 1.85:1 aspect ratio, such as the U.S.-Italian co-production Man of La Mancha, fit quite comfortably onto a 1.78:1 HDTV screen and have been issued anamorphically enhanced on DVD without the black bars. Note that, in order for this to work, the 'pixels' are assumed to not be square; but have a 1:1.094 ratio.
Why 16:9 was chosen by the SMPTE
When the 16:9 aspect ratio was proposed by Dr. Kerns H. Powers, a member of the SMPTE Working Group On High-Definition Electronic Production, nobody was creating 16:9 videos. The popular choices in 1980 were: 4:3 (based on television standard's ratio at the time), 1.66:1 (the European "flat" ratio), 1.85:1 (the American "flat" ratio), 2.20:1 (the ratio of 70 mm films and Panavision) and 2.35:1 (the CinemaScope ratio for anamorphic widescreen films). Dr. Powers cut out rectangles with equal areas and shaped them to match each of the popular aspect ratios. When overlapped with their center points aligned, he found that all of those aspect ratio rectangles fit within an outer rectangle with an aspect ratio of 1.77:1 and all of them also covered a smaller common inner rectangle with the same aspect ratio 1.77:1. The value found by Powers is exactly the geometric mean of the extreme aspect ratios, 4:3 (1.33:1) and 2.35:1, which is coincidentally close to 16:9 (1.78:1). The inclusion of the intermediate ratios into consideration is pointless, as they can have no effect on the result.
While 16:9 (1.78:1) was initially selected as a compromise format, the subsequent popularity of HDTV broadcast has solidified 16:9 as perhaps the most important video aspect ratio for the future. Most 4:3 (1.33:1) and 2.39:1 video is now recorded using a "shoot and protect" technique that keeps the main action within a 16:9 (1.78:1) inner rectangle to facilitate HD broadcast.. Conversely it is quite common to use a technique known as center-cutting, to approach the challenge of presenting material shot (typically 16:9) to both a HD and legacy 4:3 audience simultaneously without having to compromise image size for either audience. Content creators frame critical content or graphics to fit within the 1.33 raster space. Audiences generally do not see such centrally framed information as distracting. However, audiences of 16:9 ratio scenes can find odd moving elements that are centrally framed. 4:3 content upconverted to a 16:9 standard is generally referred to as pillar boxed and many high definition television networks have adopted decoratively branded logos to fill the null area.
After the original 16:9 Action Plan of the early 1990s, the European Union has instituted the 16:9 Action Plan, just to accelerate the development of the advanced television services in 16:9 aspect ratio, both in PAL and also in HDTV. The Community fund for the 16:9 Action Plan amounted to €228 million.
16:9 in Europe
In Europe, 16:9 is being adopted as the standard broadcast format for digital and high definition TV. Some countries have even adopted the format for analog television by means of the PALplus standard.