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. 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 (2.370370) 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. 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).
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.