Aspect Ratio : Digital Video and HD

Aspect ratio for image or video, is the ratio of an image’s width to its height. Standard aspect ratios for film and video are sketched. Aspect ratio is sometimes called display aspect ratio (DAR) or picture aspect ratio (PAR). Standard-definition (SD) television has an aspect ratio of 4:3

DAR <== Digital Aspect Ratio
PAR <== Picture Aspect Ratio
SD <== Standard-Definition
HD <== High Deffinition

Cinema film commonly uses 1.85:1 (which for historical reasons is called either flat or spherical), or 2.4:1 (“CinemaScope.” or colloquially, scope). Many films are 1.85:1, but “blockbusters” are usually 2.4:1. Film at 2.4:1 aspect ratio was historically acquired using an aspherical lens that squeezes the horizontal dimension of the image by a factor of two. The projector is equipped with a similar lens, to restore the horizontal dimension of the projected image. The lens and the technique are called anamorphic. In principle, an anamorphic lens can have any ratio, in practice; a ratio of exactly two is ubiquitous in cinema.

Widescreen refers to an aspect ratio wider than 4:3 High-definition (HD) television is standardized with an aspect ratio of 16:9 widescreen variant of a base video standard, where the horizontal dimension of the 16:9 image occupies the same width as the 4:3 aspect ratio standard. Consumer electronic equipment rarely recovers the correct aspect ratio of such conversions.

Standard Definition 4 : 3 12 : 9
High Definition 16 : 9 16 : 9
Cinema File 1.85 : 1 16.65 : 9
Blockbusters 1.85 : 1 21.6 : 9

(If the same high, Blockbusters has the longest width)

HD is standardized with an aspect ratio of 16:9 (about 1.78:1), fairly close to the 1.85:1 ordinary movie aspect ratio Figure 1.3 below illustrates the origin of the 16:9 aspect ratio. Through a numerological coincidence apparently first revealed by Kerns Powers, the Geometric mean of 4:3 (the standard aspect ratio of conventional television) and 2.4 (the aspect ratio of a CinemaScope movie) is very close within a fraction of a percent to 16:9 (The calculation is shown in the lower right corner of the figure.) A choice of 16:9 for HD meant that SD, HD, and CinemaScope shared the same “image circle”: 16:9 was a compromise between the vertical cropping required for SD and the horizontal cropping required for CinemaScope.


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