Aspect ratio (image)
The aspect ratio of an image is the ratio of its width to its height. It is expressed as two numbers separated by a colon, width:height. Common aspect ratios are 1.85:1 and 2.40:1 in cinematography, 4:3 and 16:9 in television, and 3:2 in still photography.
This article is about aspect ratio of still images, films, and digital imagery. For display aspect ratio, see display aspect ratio.Practical limitations[edit]
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 1.33:1.[3]
With a space designated for the standard optical soundtrack, and the frame size reduced to maintain an image that is wider than tall; this resulted in the Academy aperture of 22 mm × 16 mm (0.866 in × 0.630 in) or 1.375:1 aspect ratio.
Current video standards[edit]
1:1[edit]
Square displays are rarely used in devices[6][7] and monitors.[8] Nonetheless, video consumption on social apps has grown rapidly and led to the emergence of new video formats more suited to mobile devices that can be held in horizontal and vertical orientations. In that sense, square video was popularized by mobile apps such as Instagram and Vine and has since been supported by other major social platforms including Facebook and Twitter. It can fill nearly twice as much screen space compared to 16:9 format (when the device is held differently while viewing from how video was recorded).
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 numbers of pixels. If an image is displayed with square pixels, then these ratios agree. If, instead, non-square ("rectangular") pixels are used, then these ratios differ. 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:
Rearranging (solving for PAR) yields:
For example:
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 PAR 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; PAR is also known as sample aspect ratio, though it can also be confused with storage aspect ratio.