Flikipedia A branching dictionary of the moving image

aspect ratio

noun

The ratio of an image's width to its height. In the moving image it fixes the shape of the frame, and is written either as a ratio normalized to a height of 1 (such as 1.85:1) or as two whole numbers (such as 16:9).

Origin From aspect (Latin aspectus, "a looking at, appearance") and ratio (Latin, "reckoning, proportion"); the compound came into technical use in engineering and photography for the proportion of a rectangle's sides.

  1. Optics The proportion of image width to image height, expressed as width divided by height. A value of 1.85:1 means the picture is 1.85 times as wide as it is tall; a value of 1:1 is square.

  2. Format The corresponding proportion of a physical or storage format — the shape of a film frame on the negative, of the projected image on screen, or of the pixel array of a digital file or display. a standard-definition television frame at 4:3

Reading the numbers

An aspect ratio compares the width of an image to its height. It is most often written normalized to a height of one — 1.33:1, 1.85:1, 2.39:1 — so that the first number alone gives the relative width. The same shape can also be written as a pair of whole numbers, which is common in video: 4:3 and 16:9 are equal to 1.33:1 and 1.78:1 respectively. The ratio is a property of the frame — the shape of the single image — and so governs how much of a scene the lens and camera take in horizontally relative to vertically, independent of the frame’s absolute size.

Ratios of the moving image

Early silent film settled on roughly 1.33:1, matching the 4:3 shape of the 35mm frame between the perforations. When an optical soundtrack was added alongside the image in the late 1920s, it encroached on the frame; the Academy ratio of 1.375:1 was standardized in 1932 to restore an approximately rectangular picture with sound. From the 1950s, theatrical film adopted wider shapes to distinguish cinema from television: 1.85:1 became the common “flat” widescreen standard, and 2.39:1 (earlier specified as 2.35:1) became the standard for anamorphic “scope” presentation. Video moved from the 4:3 shape of standard-definition television to 16:9 (1.78:1) with high-definition, a compromise ratio positioned between the older television and the widest theatrical formats.

Masking and anamorphic lenses

A wide ratio can be achieved in two ways. The first is masking (also called a “flat” or spherical process): the image is recorded with an ordinary spherical lens, and the top and bottom of the frame are hidden — by an aperture plate in the camera, by a matte, or by a projector mask — leaving a wide strip. This is how 1.85:1 is typically produced, and it discards part of the recorded image height. The second is anamorphic: a special lens optically squeezes a wide field of view horizontally onto the frame, so the recorded image looks compressed. A matching lens on the projector, or a stretch in processing, unsqueezes it to the wide 2.39:1 shape. Anamorphic uses the full frame height rather than masking it away, and its optics produce distinctive artifacts such as oval out-of-focus highlights and horizontal lens flares.

Framing consequences

Because the ratio sets the shape of the frame before anything is placed within it, it is a fundamental parameter in cinematography. Presenting an image made for one ratio in another requires either letterboxing or pillarboxing (adding black bars to preserve the original shape), cropping, or pan and scan (selecting a moving window of the wider image). Each alters what the viewer sees relative to what was composed, which is why the aspect ratio is recorded as part of a work’s specification and preserved in archival and home-video transfers.

See also

  • lens— The optical element or assembly that gathers light and forms an image on photographic film or an electronic sensor, characterized chiefly by its focal length and its maximum aperture.
  • frame— A single still image in a film strip or digital video stream; the basic unit of the moving image, from which a motion picture is built by rapid succession.
  • cinematography— The art and craft of capturing the images of a motion picture: the cinematographer's work of choosing lens, exposure, lighting, framing, and camera movement to render a film visually.

Sources

  1. SMPTE (Society of Motion Picture and Television Engineers) — projection and image standards
  2. American Cinematographer Manual, ASC Press
  3. John Belton, "Widescreen Cinema" (Harvard University Press, 1992)

Last revised: September 18, 2026