Aspect ratio (image)

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Aspect ratio (image)
 Five common aspect ratios

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"), with the latter particularly used for pixel dimensions, such as 640×480. Cinematographic aspect ratios are usually denoted as a (rounded) decimal multiple of width vs unit height, while photographic and videographic aspect ratios are usually defined and denoted by whole number ratios of width to height. In digital images there is a subtle distinction between the Display Aspect Ratio (the image as displayed) and the Storage Aspect Ratio (the ratio of pixel dimensions); see distinctions, below.

The most common aspect ratios used today in the presentation of films in movie theaters are 1.85:1 and 2.39:1.[1] Two common videographic aspect ratios are 4:3 (1.33:1), the universal video format of the 20th century and ; 16:9 (1.77:1), universal for high-definition television and European digital television. Other cinema and video aspect ratios exist, but are used infrequently. As of 2010, nominally 21:9 (2.33) aspect TVs have been introduced by Philips and Vizio (the latter using an LCD from AU Optronics) as "cinema" displays, though the resolution is more precisely 2560 / 1080 = 2.37 (2.370) exactly), and the aspect ratio is not standardized in HDTV.

In still camera photography, the most common aspect ratios are 4:3, 3:2, and more recently being found in consumer cameras 16:9.[2] Other aspect ratios, such as 5:4, 6:7, and 1:1 (square format), are used in photography as well, particularly in medium format and large format.

With television, DVD and Blu-ray, converting formats of unequal ratios is achieved by either: enlarging the original image (by the same factor in both directions) 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 (hence distorting) the image to fill the receiving format's ratio, 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).

Practical limitations

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.[3]

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.

Cinema terminology

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 in 1.66:1). During that time, television, which had a similar aspect ratio of 1.3:1, became a threat to movie audiences, Hollywood gave birth to 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 1.66:1 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.

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 is 1.66:1 while 16:9 is 1.7:1). It also can be blown up to 35 mm for theatrical release and therefore is also used for feature films.

Current video standards

4:3 standard

4:3 (1.33:1) (generally named as "Four-Three", "Four-by-Three", and "Four-to-Three") for standard television has been in use since television's origins and many computer monitors employ the same aspect ratio. 4:3 is the aspect ratio used for 35 mm films in the silent era and used today for film production under the name Super 35. It is also very close to the 1.37:1 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 standard

16:9 (1.77:1) (generally named as "Sixteen-Nine", "Sixteen-by-Nine" and "Sixteen-to-Nine") is the international standard format of HDTV, non-HD digital television and analog widescreen television 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, and 16:9 is the only widescreen aspect ratio natively supported by the DVD. DVD producers can also choose to show even wider ratios such as 1.85:1 and 2.39:1[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 1.77:1 HDTV screen and have been issued anamorphically enhanced on DVD without the black bars.

Obtaining height, width and area of the screen

Often, screen specifications are given by their diagonal length. The following formulae can be used to find the height (h), width (l for length) and area (A), where r stands for ratio and d for diagonal length.

$h=\frac{d}{\sqrt{r^2+1}} \qquad l=\frac{d}{\sqrt{\frac{1}{r^2}+1}} \qquad A=\frac{d^2}{r+\frac{1}{r}}$

Distinctions

This article primarily addresses the aspect ratio of images as displayed, which is more formally referred to as the Display Aspect Ratio (DAR). In digital images, there is a distinction with the Storage Aspect Ratio (SAR), which is the ratio of pixel dimensions. If an image is displayed with square pixels, then these ratios agree; if not, then non-square, "rectangular" pixels are used, and these ratios disagree. The aspect ratio of the pixels themselves is known as the Pixel Aspect Ratio (PAR) – for square pixels this is 1:1 – and these are related by the identity:

SAR × PAR = DAR.

Rearranging (solving for PAR) yields:

PAR = DAR/SAR.

For example, a 640 × 480 VGA image has a SAR of 640/480 = 4:3, and if displayed on a 4:3 display (DAR = 4:3) has square pixels, hence a PAR of 1:1. By contrast, a 720 × 576 D-1 PAL image has a SAR of 720/576 = 5:4, but is displayed on a 4:3 display (DAR = 4:3), so by this formula it would have a PAR of (4:3)/(5:4) = 16:15.

However, because standard definition digital video was originally based on digitally sampling analog television, the 720 horizontal pixels actually capture a slightly wider image to avoid loss of the original analog picture. In actual images, these extra pixels are often partly or entirely black, as only the center 704 horizontal pixels carry actual 4:3 or 16:9 image. Hence, the actual pixel aspect ratio for PAL video is a little different from that given by the formula, specifically 12:11 for PAL and 10:11 for NTSC. For consistency, the same effective pixel aspect ratios are used even for standard definition digital video originated in digital form rather than converted from analog. For more details refer to the main article.

In analog images such as film there is no notion of pixel, nor notion of SAR or PAR, and "aspect ratio" refers unambiguously to DAR. Actual displays do not generally have non-square pixels, though digital sensors might; they are rather a mathematical abstraction used in resampling images to convert between resolutions.

Non-square pixels arise often in early digital TV standards, related to digitalization of analog TV signals – whose horizontal and vertical resolutions differ and are thus best described by non-square pixels – and also in some digital videocameras and computer display modes, such as Color Graphics Adapter (CGA). Today they arise particularly in transcoding between resolutions with different SARs.

DAR is also known as Image Aspect Ratio and Picture Aspect Ratio, though the latter can be confused with Pixel Aspect Ratio.

Visual comparisons

Comparing two different aspect ratios poses some subtleties – when comparing two aspect ratios, one may compare images with equal height, equal width, equal diagonal, or equal area. More amorphous questions include whether particular subject matter has a natural aspect ratio (panoramas being wide, full-length images of people being tall), or whether a particular ratio is more or less aesthetically pleasing – the golden ratio (1.618) is seen as especially pleasing, though only the 16:10 format is close to it.

Given the same diagonal, the 4:3 screen offers more area, because it is closer to square (which maximizes area given a diagonal). For CRT-based technology, an aspect ratio that is closer to square is cheaper to manufacture. The same is true for projectors, and other optical devices such as cameras, camcorders, etc. For LCD and Plasma displays, however, the cost is more related to the area, so producing wider and shorter screens yields the same advertised diagonal but lower area, and hence is more profitable.

The following compares crops of a given image at 4:3 and 16:9, with different parameters equal; note that in terms of subject, the squarer aspect ratio emphasizes the public square, while the wider aspect ratio emphasizes the wide building.

• Two aspect ratios compared with images using the same diagonal size:
 4:3 (1.33:1) 16:9 (1.77:1)
• Two aspect ratios compared with images using the same area (number of pixels):
 4:3 (1.33:1) 16:9 (1.77:1)
• Two aspect ratios compared with images using the same height (vertical size):
 4:3 (1.33:1) 16:9 (1.77:1)
• Two aspect ratios compared with images using the same width (horizontal size):
 4:3 (1.33:1) 16:9 (1.77:1)

Previous and currently used aspect ratios

See list of common resolutions for a listing of computer resolutions and aspect ratios.
See list of film formats for a full listing of film formats, including their aspect ratios.
Comparison between several film aspect ratios with the heights forced to be equal.
1.15
Sometimes referred to as the Movietone ratio, this ratio was used briefly during the transitional period when the film industry was converting to sound, from 1926-32 approx. It is produced by superimposing an optical soundtrack over a full-gate 1.3 aperture in printing, resulting in an almost square image. Films shot in this ratio are often projected or transferred to video incorrectly using a 1.37 mask. Examples of films shot in the Movietone ratio include Sunrise, M and Hallelujah!.[4]
1.3 (4:3)
35 mm original silent film ratio, today commonly known in TV and video as 4:3. Also standard ratio for MPEG-2 video compression. This format is still used in many personal video cameras today and has influenced the selection or design of other aspect ratios. It is the standard 16 mm and Super 35mm ratio.
1.37
35 mm full-screen sound film image, nearly universal in movies between 1932 and 1953. Officially adopted as the Academy ratio in 1932 by AMPAS. Rarely used in theatrical context nowadays, but occasionally used for other context.
1.43
IMAX format. Imax productions use 70 mm wide film (the same as used for 70 mm feature films), but the film runs through the camera and projector horizontally. This allows for a physically larger area for each image.
1.50 (3:2)
The aspect ratio of 35 mm film used for still photography when 8 perforations are exposed. Also the native aspect ratio of VistaVision, for which the film runs horizontally.
1.5 (14:9)
Widescreen aspect ratio sometimes used in shooting commercials etc. as a compromise format between 4:3 (12:9) and 16:9. When converted to a 16:9 frame, there is slight pillarboxing, while conversion to 4:3 creates slight letterboxing.
1.60 (16:10)
Widescreen computer monitor ratio (for instance 1920x1200 resolution).
1.6 (15:9 = 5:3)
35 mm Originally a flat ratio invented by Paramount Pictures, now a standard among several European countries; native Super 16 mm frame ratio. Sometimes this ratio is rounded up to 1.67:1. From the late 1980's to the early 2000's, Walt Disney Feature Animation's CAPS program animated their features in the 1.66:1 ratio (a compromise between the 1.85:1 theatrical ratio and the 1.33:1 ratio used for home video).
1.75
Early 35 mm widescreen ratio, primarily used by MGM and Warner Bros. between 1953 and 1955, and since abandoned, though Disney has cropped some of it's post-50's Full Screen films to this ratio for DVD i.e. The Jungle Book.
1.7 (16:9 = 42:32)
Video widescreen standard, used in high-definition television, one of three ratios specified for MPEG-2 video compression. Also used increasingly in personal video cameras. Sometimes this ratio is rounded up to 1.78:1.
1.85
35 mm US and UK widescreen standard for theatrical film. Introduced by Universal Pictures in May, 1953. Projects approximately 3 perforations ("perfs") of image space per 4 perf frame; films can be shot in 3-perf to save cost of film stock.
2.00
Original SuperScope ratio, also used in Univisium. Used as a flat ratio for some American studios in the 1950s, abandoned in the 1960s, but recently popularized by the Red One camera system. In 2001 Studio Ghibli used this framing for it's animated film Spirited Away.
2.10
Planned futuristic aspect ratio for television and theatrical films.
2.20
70 mm standard. Originally developed for Todd-AO in the 1950s.
2.21
Specified in MPEG-2 for 2.20:1 movies, but hardly used.
2.35
35 mm anamorphic prior to 1970, used by CinemaScope ("'Scope") and early Panavision. The anamorphic standard has subtly changed so that modern anamorphic productions are actually 2.39,[1] but often referred to as 2.35 anyway, due to old convention. (Note that anamorphic refers to the compression of the image on film to maximize an area slightly taller than standard 4-perf Academy aperture, but presents the widest of aspect ratios.) All Indian Bollywood films released after 1972 are shot in this standard for theatrical exhibition.
2.37 (64:27 = 43:33)
As of 2010, TVs have been introduced with this aspect ratio and are marketed as "21:9 cinema displays". This aspect ratio is not recognized by storage and transmission standards.
2.40
35 mm anamorphic from 1970 onwards. Aspect ratio of current anamorphic (wide-screen) theatrical viewings. 2.39:1 is sometimes rounded up to 2.40:1.[1] Often commercially branded as Panavision format or 'Scope.
2.55
Original aspect ratio of CinemaScope before optical sound was added to the film in 1954. This was also the aspect ratio of CinemaScope 55.
2.59
Cinerama at full height (three specially captured 35 mm images projected side-by-side into one composite widescreen image).
2.6
Full frame output from Super 16 mm negative when an anamorphic lens system has been used. Effectively, an image that is of the ratio 2.6:1 is squashed onto the native 15:9 aspect ratio of a Super 16 mm negative.
2.76
Ultra Panavision 70 (65 mm with 1.25x anamorphic squeeze). Used only on a handful of films between 1962 and 1966, such as the Battle of the Bulge (1965).
2.93
MGM Camera 65, an early version of Ultra Panavision used up until 1962 which used a 1.33x anamorphic squeeze instead to produce a wider aspect ratio. Used only on a few early Ultra Panavision films, most notably Ben-Hur (1959) and also for some sequences of How The West was Won with a slight crop to 2.89:1 when converted to three strip Cinerama.
4.00 (4:1 = 4×1
1): Rare use of Polyvision, three 35 mm 1.33 (1:1) images projected side by side. First used in 1927 on Abel Gance's Napoléon.
12.0
Circle-Vision 360° developed by the Walt Disney Company in 1955 for use in Disneyland. Uses (9)-4:3 35mm projectors to show an image that completely surrounds the viewer. Used in subsequent Disney theme parks to this day, and other past applications.

Aspect ratio releases

Original aspect ratio (OAR)

Original Aspect Ratio (OAR) is a home cinema term for the aspect ratio or dimensions in which a film or visual production was produced — as envisioned by the people involved in the creation of the work. As an example, the film Gladiator was released to theaters in the 2.39:1 aspect ratio. It was filmed in Super 35 and, in addition to being presented in cinemas and television in the Original Aspect Ratio of 2.39:1, it was also broadcast without the matte altering the aspect ratio to the television standard of 1.33:1. Because of the varied ways in which films are shot, IAR (Intended Aspect Ratio) is a more appropriate term, but is rarely used.

Modified aspect ratio (MAR)

Modified Aspect Ratio is a home cinema term for the aspect ratio or dimensions in which a film was modified to fit a specific type of screen, as opposed to original aspect ratio. Modified aspect ratios are usually either 1.33:1 (historically), or (with the advent of widescreen television sets) 1.77:1 aspect ratio. 1.33:1 is the modified aspect ratio used historically in VHS format. A modified aspect ratio transfer is achieved by means of pan and scan or open matte, the latter meaning removing the cinematic matte from a 1.85:1 film to open up the full 1.33:1 frame. Another name for it is "prescaled" aspect ratio".

Problems in film and television

A windowboxed image

Multiple aspect ratios create additional burdens on filmmakers and consumers, and confusion among TV broadcasters. It is common for a widescreen film to be presented in an altered format (cropped, letterboxed or expanded beyond the Original Aspect Ratio). It is also not uncommon for windowboxing to occur (when letterbox and pillarbox happen simultaneously). For instance, a 16:9 broadcast could embed a 4:3 commercial within the 16:9 image area. A viewer watching on a standard 4:3 (non-widescreen) television would see a 4:3 image of the commercial with 2 sets of black stripes, vertical and horizontal (windowboxing or the postage stamp effect). A similar scenario may also occur for a widescreen set owner when viewing 16:9 material embedded in a 4:3 frame, and then watching that in 16:9. Active Format Description is a mechanism used in digital broadcasting to avoid this problem. It is also common that a 4:3 image is stretched horizontally to fit a 16:9 screen to avoid pillar boxing but distorts the image so subjects appear short and fat.

Both PAL and NTSC have provision for some data pulses contained within the video signal used to signal the aspect ratio (See ITU-R BT.1119-1 - Widescreen signaling for broadcasting). These pulses are detected by television sets that have widescreen displays and cause the television to automatically switch to 16:9 display mode. When 4:3 material is included (such as the aforementioned commercial), the television switches to a 4:3 display mode to correctly display the material. Where a video signal is transmitted via a European SCART connection, one of the status lines is used to signal 16:9 material as well.

Still photography

Common aspect ratios in still photography include 4:3 (1.33) used by most digital point-and-shoot cameras, Four Thirds system cameras and medium format 645 cameras; 3:2 (1.5) used by 35 mm film, APS-C ("classic" mode) and most DSLRs; 1.81:1 (close to 16:9) used by APS-H high definition mode and some Panasonic multi‐aspect Four Thirds and compact cameras; 3:1 used by APS‐P panoramic mode; and 1:1 (square) in a variety of cameras. Many digital cameras offer user options for selecting image aspect ratio, but all this function does is reject the storage of rectangular edge pixel blocks from the native sensor resolution.

The reason for DSLR image sensors being the flatter 3:2 versus the taller point-and-shoot 4:3 is that DSLRs were designed to match the legacy 35 mm SLR film, whereas the majority of digital cameras were designed to match the predominant computer displays of the time, with VGA, SVGA, XGA and UXGA all being 4:3. (Widescreen computer monitors did not become popular until the advent of HDTV.)

Common print sizes in the U.S. (in inches) include 4×6 (1.5), 5×7 (1.4), 4×5 and 8×10 (1.25), and 11×14 (1.27); large format cameras typically use one of these aspect ratios. Medium-format cameras typically have format designated by nominal sizes in centimeters (6×6, 6×7, 6×9, 6×4.5), but these numbers should not be interpreted as exact in computing aspect ratios. Print sizes are usually defined by their portrait dimensions (tall) while equipment aspect ratios are defined by their landscape dimensions (wide, flipped sideways). A good example of this a 4×6 print (6 inch wide by 4 inch tall landscape) perfectly matches the 3:2 aspect ratio of a DSLR/35 mm, since 6/2=3 and 4/2=2.

For analog projection of photographic slides, projector and screen use a 1:1 aspect ratio, supporting horizontal and vertical orientation equally well. In contrast, digital projection technology typically supports vertically oriented images only at a fraction of the resolution of landscape-oriented images. For example, projecting a digital still image having a 3:2 aspect ratio on a 16:9 projector employs 84.3% of available resolution in horizontal orientation, but only 37.5% in vertical orientation.

References

Cited references
1. ^ a b c d The 2.39:1 ratio is commonly labeled 2.40:1, e.g., in the American Society of Cinematographers' American Cinematographer Manual, and is mistakenly referred to as 2.35:1 (only cinema films before the 1970 SMPTE revision used 2.35:1).
2. ^
3. ^ Burum, Steped (2004). American Cinematographer Manual (9th ed.). ASC Press. ISBN 0-935578-24-2
4. ^ Scott Eyman, The Speed of Sound: Hollywood and the Talkie Revolution, 1926-1930, New York, Simon & Schuster (1997), p. 222.
General references

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