# Aspect ratio

Under aspect ratio in a broad sense refers to the ratio of at least two different long sides of a polygon . Most of the time it is the ratio of the width of a rectangle to its height. A square has an aspect ratio of 1: 1. In the video sector , the aspect ratio is also used in English .

The aspect ratio width to height is specified in the format N: M , but in the cinema M: N , i.e. height to width

The aspect ratio is often specified on screens as a fraction N: M (e.g. 16: 9 ), this fraction is often normalized to 1 and possibly rounded (e.g. 1.78: 1).

## photography

Countless formats with a wide variety of aspect ratios are used in photography on film material. The most widespread formats were 3: 2 ( 35mm film 135 with 24 × 36 mm, medium format - roll film 120/220 with 6 × 9 cm) and the square 1: 1 (Instamatic 126 with 28 × 28 mm, roll film with 6 × 6 cm). The Pocket Instamatic films 110 with 13 × 17 mm and the "Classic" format APS-C with 16.7 × 23.4 mm , which were formerly widespread in the amateur sector, are not exactly exactly, but close to the ratio 3: 2 . With the APS film, the 16: 9 format based on the HDTV system was introduced into film-based photography, as well as a panorama format with an aspect ratio of 3: 1.

The roll film also serves as the basis for several other formats, including panorama cameras with the formats 6 × 17 cm or 6 × 20 cm. However, only the format 67 with 6 × 7 cm and in particular the "645" with 4.5 × 6 cm, with which the 4: 3 aspect ratio became popular, which is still common today in digital medium format, was only widely used.

In large format photography, aspect ratios range from 1.25: 1 (4 × 5 inches and 8 × 10 inches), 1.4: 1 (5 × 7 inches), 1.27: 1 (11 × 14 inches) up to to 2.5: 1 (4 × 10 inches) common, as well as in the metric system 1.25: 1 (9 × 12 cm, (approx.) 13 × 18 cm, 18 × 24 cm).

The photo papers used for exposure in the mass market traditionally always had a slightly shorter aspect ratio than the common cameras in 3: 2 format, since the images were usually provided with a border. With the formerly common formats of 7 × 10 cm and 9 × 13 cm, the aspect ratio used by the film was exactly the same as the aspect ratio used by the film with a margin of approx. When borderless enlargement techniques were introduced, the paper formats were initially retained, with the side effect that the enlarged photos were trimmed more on the narrow sides than on the long sides; the sheets were "too short". The problem was resolved with the introduction of the so-called king-size formats, e.g. 10 × 15 cm for 35mm film. Photo papers that are made up as sheet goods often also have aspect ratios smaller than 3: 2, they are usually in the range between 1.2: 1 and 1.4: 1, including DIN formats (“postcard” 10.5 × 14 , 8 cm) occur.

The aspect ratios traditionally used in film have been adopted in digital photography. Most digital single lens reflex (DSLR) cameras take pictures with an aspect ratio of 3: 2. Four-thirds standard cameras and most digital compact cameras, on the other hand, use the 4: 3 format - most medium format cameras use the same format . The 16: 9 aspect ratio has also been added, especially with video-capable digital cameras.

## Movie

Comparison of the three most common aspect ratios: The outer frame (blue) and the middle frame (red) are the most popular formats for film recordings, the inner green frame is the (former) standard 4: 3 format for television

As the film developed , there were always new image formats, but most of them sooner or later went out of style. Others have been preserved to this day. In the cinema, the picture height (screen height) usually always remains the same - an exception are e.g. B. cinemas suitable for showing 70 mm film, which usually cover the lowest part of the screen. The image width varies depending on the film format. That is why in the cinema - unlike in video, where the width is the reference dimension - the height is always mentioned first and the width behind.

The classic aspect ratio of 35 mm film is 4 to 3. This aspect ratio is also used in amateur films. In feature films, widescreen formats with the ratios 1.66: 1 (5: 3) and 1.85: 1 (37:20) are used more and more frequently . These relationships are generated by corresponding image windows in the film camera or masks in the projector, which are inserted into the beam path. So a smaller section of each film frame is simply used, and this is then stretched proportionally. (Further explanation → see under Open Matte )

With the anamorphic method one goes a different way. The original ratio of 2.39: 1 (55:23, previously 2.35: 1) is applied to the film material with the help of an anamorphic lens, distorted by a factor of 2. The height of the image is retained. When the finished film is shown, the image is rectified with a factor 2 anamorphic lens.

In the 1950s and early 1960s, different methods and aspect ratios were experimented with. A special procedure was VistaVision (1954), for which new cameras were required, which recorded on horizontally guided 35 mm film. The aspect ratio was 2: 1 on vertical cinema copies.

The widest film formats are the Cinerama , which was used from 1952 to 1962, with an aspect ratio of 2.65: 1, and the Ultra Panavision 70, which has been very rarely used since 1957, with a ratio of 2.76: 1.

## Television and video

In analog television (originally black and white), 4: 3 was the uniform format for decades, as was the case with German PAL as well as French SECAM and the US NTSC color television. From the 1990s onwards, the 16: 9 format (= 1.78: 1) was used with increasing frequency .

The digital television standards such as DVB and ATSC support the display aspect ratios 16: 9 and 4: 3 as well as theoretically 2.21: 1 (about 20: 9), which is not used in practice , for a large number of pixel aspect ratios . When high-definition television (HDTV) aspect ratio is 16: 9 common.

With DVD , SVCD and DVB the images are often saved anamorphically - analogous to the CinemaScope process.

With digitization and the abandonment of the cathode ray tube (CRT), the 16: 9 aspect ratio prevailed, especially for larger, high-quality televisions . From 2009 even wider devices in the format 21: 9 (correct 64:27 = 2.37: 1) came onto the market, which can display cinema films in the format 2.39: 1 without horizontal stripes, whereby no optimized source material exists and Most content has vertical bars, inflated or distorted display.

Screen aspect ratios
21: 16: 15: 14: 12:
:8th 2: 1 3: 2
: 9 64:27 16: 9 5: 3 14: 9 4: 3
: 10 8: 5 3: 2
: 12 4: 3 5: 4 1: 1

## Other formats

• √2Φ : 1 - The aspect ratio for A4 sheets and related DIN dimensions. Dividing the rectangle by halving the longer side results in two rectangles with the same aspect ratio (≈1.4142: 1)
• Φ √0: 1 - aspect ratio in the golden ratio (≈1.618: 1)
• 5 Φ√: 3 - Used as a film format alongside the even wider 1.85: 1 (corresponds to 15: 9). PDAs and comparable devices of the more recent times, mostly equipped with network and video capabilities as well as a touchscreen , are now also using it, e.g. B. with screen resolutions of 800 × 480. Among other things, it is also the aspect ratio of the flags of Germany , Liechtenstein and Luxembourg

## Display aspect ratio

The display aspect ratio ( English Display Aspect Ratio , DAR ) are initially only the ratio of the width to the height (in units of length, about cm) of the image on the output medium. For an image to be displayed, however, it is often used to describe the ratio of the width to the height of the part of the output medium on which it is to be displayed (without distortion) (correct original aspect ratio or OAR, see below).

## Pixel aspect ratio

The pixel aspect ratio ( English pixel aspect ratio , PAR ) is the ratio of width to height of a single pixel (pixel) of the output medium on.

On computer screens, pixels are square (PAR 1: 1) as standard, on (analog) TV screens they are - for historical reasons - rectangular, with PAL 4: 3 exactly 128/117 = approx. 1.094, i.e. slightly wider than high.

## conversion

The following mathematical relationship applies to digitally coded video:

{\ displaystyle {\ begin {aligned} {\ mathit {PAR}} & = {\ frac {\ mathit {DAR}} {\ mathit {SAR}}} \\ {\ frac {x_ {p}} {y_ { p}}} & = {\ frac {\ frac {x_ {d}} {y_ {d}}} {\ frac {x_ {s}} {y_ {s}}}} = {\ frac {x_ {d }} {y_ {d}}} \ times {\ frac {y_ {s}} {x_ {s}}} = {\ frac {x_ {d}} {x_ {s}}} \ times {\ frac { y_ {s}} {y_ {d}}} \ end {aligned}}}
PAR = pixel aspect ratio
x / y ratio of a single pixel
DAR = display aspect ratio
x / y ratio of the image to be displayed, e.g. B. 4/3 or 16/9
SAR = storage aspect ratio
x / y ratio of the stored resolution (number of pixels), e.g. B. 720/576

The pixels are often on the media squarely stored, d. H. PAR = 1: 1; the present aspect ratio of the image (DAR) with these square pixels stored is therefore identical to SAR. Important exceptions to the use of square pixels are DVD and (normal definition) DVB .

DAR is the required aspect ratio (that of the monitor or, if not identical, just the file) that should come out at the end, mostly corresponding to the display monitors 4: 3 or 16: 9. PAR is then the ratio by which each pixel must be stretched horizontally (with PAL) or vertically (with NTSC) in order to obtain the required aspect ratio DAR; this makes all pixels non-square in the display. Square and non-square always refer to the shape of the individual pixel, not to the shape of the overall image.

The non-square resolutions 720 × 576 (PAL, which corresponds to a ratio of 5: 4 instead of 4: 3) and 720 × 480 (NTSC) have historical roots; To compensate for this, the square 768 × 576 is sometimes used in the A / D conversion , the same non-square analog image point being represented by parts of several square pixels. Since the advent of the DVD, however, non-square pixel aspect ratios have also been used preferentially to increase the vertical resolution of widescreen films without increasing the storage or bandwidth requirements; by anamorphic vertical compression (instead of stretching) the pixels stored in the 5: 4 ratio on the medium to 16: 9, an undistorted display with a smaller height in relation to the width is achieved.

The scaling to the correct DAR takes place in the case of display / decoding. So that the video decoder can calculate the necessary scaling, either the DAR ( MPEG2 ) or PAR ( MPEG-4 ) is saved in the data stream. If this attribute is ignored during decoding, distortions can occur. Often the display software ( video player ) also supports the overwriting of the attribute, because in certain cases it is already incorrect or not contained in the data stream. ${\ displaystyle {\ mathit {PAR}} \ neq 1}$

## Original aspect ratio

With the original aspect ratio ( English Original Aspect Ratio , OAR ) the director intended to let the audience see his film in the original aspect ratio. When transferring images from a movie to a DVD or TV film, the aspect ratio is not changed, but remains unchanged (2.35: 1, 1.85: 1 or 1.66: 1). Since the aspect ratio for television broadcasting in PAL and NTSC is fixed at 1.33: 1 (4: 3), when a cinema film is broadcast with OAR in PAL / NTSC, the "empty" area is filled in black so that above and below black bars appear in the image - the so-called letterbox , also mockingly called “mourning bars ” by some. In the case of an anamorphic appearance , the image is subsequently compressed into the correct aspect ratio, which also results in black bars. In contrast to this is the pan-and-scan process, in which parts of the image are cut off at the sides in order to fill the entire height of the television screen with image.