Flikipedia A branching dictionary of the moving image

exposure

/ɪkˈspoʊʒər/ noun

The quantity of light reaching the film or sensor while an image is recorded, and the act of admitting it — governed jointly by aperture, shutter, and sensitivity.

Origin From Latin exponere, "to set forth or lay open," via the sense of laying a sensitive surface open to light. In photography the term dates to the mid-19th century.

  1. Photometry The total light received by the photographic film or image sensor during the recording of a frame, commonly measured in lux-seconds — the product of the illuminance at the medium and the time it is exposed.

  2. Craft The act or duration of admitting light to the medium; by extension, a single recorded frame, as in "a long exposure."

  3. Practice The combined setting of aperture, shutter speed or angle, and sensitivity chosen to place a scene's brightness within the medium's usable range. Halving the aperture area and doubling the exposure time yield the same exposure — a reciprocal relationship known as reciprocity.

What exposure measures

Exposure is the amount of light a photographic surface receives while an image is formed — physically, the illuminance falling on the film or image sensor multiplied by the length of time it is admitted, giving a quantity in lux-seconds. Too little light yields a dark, detail-poor underexposed frame; too much yields a pale, washed-out overexposed one, in which the brightest regions “clip” and record no detail. The aim of exposure control is to place the brightnesses of a scene within the range the medium can faithfully record — its dynamic range.

The three controls

Exposure is set by three variables, often called the exposure triangle:

  • Aperture — the size of the opening in the lens, expressed as an f-number (f-stop). A smaller f-number is a wider opening admitting more light; it also narrows the depth of field.
  • Shutter — the length of time the medium is exposed each frame. In motion pictures this is often expressed as a shutter angle rather than a time; a 180-degree angle at 24 frames per second gives an exposure of about 1/48 second per frame.
  • Sensitivity — how strongly the medium responds to light, rated as film speed or, digitally, as ISO. A higher rating needs less light but typically increases grain or electronic noise.

Each control is conventionally graduated in stops: one stop doubles or halves the light. Because the three trade against one another, many combinations produce the same exposure while differing in depth of field, motion blur, and noise.

Metering

The light needed for a correct exposure is judged with a light meter, which reads scene brightness and, given a chosen film speed, indicates the aperture and shutter settings that will center the exposure. A reflected-light meter — the kind built into most cameras — measures light coming from the subject and is calibrated to an assumed average reflectance (near middle gray). An incident-light meter, held at the subject, measures the light falling on it and is unaffected by the subject’s own tone. Meters can be misled by scenes that are unusually light or dark, which is why exposure is often set relative to a known reference tone rather than to the meter’s average alone.

Reciprocity and its limits

For a wide range of settings, exposure obeys the reciprocity law: the recorded exposure depends only on the product of illuminance and time, so halving the light admitted through the aperture and doubling the shutter time leave it unchanged. At very long or very short exposure times photographic film departs from this rule — reciprocity failure — and requires extra light beyond what the simple product predicts. Digital sensors do not exhibit the same photochemical failure, though very long exposures raise thermal noise.

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.
  • shutter— The mechanism that controls how long each frame is exposed to light and blocks the light while the film advances or a sensor reads out. Its open interval, together with the frame rate, sets both exposure and the amount of motion blur recorded in each frame.
  • film speed— A measure of a photographic emulsion's sensitivity to light, expressed on the ISO scale, that sets how much exposure the film requires to record a usable image.
  • dynamic range— The span of light intensities a recording medium can register between its noise floor in the shadows and its point of saturation in the highlights, conventionally measured in photographic stops.

Sources

  1. Ansel Adams, "The Negative" (The Ansel Adams Photography Series)
  2. ISO 12232 — Photography — Digital still cameras — Determination of exposure index and ISO speed
  3. Blain Brown, "Cinematography: Theory and Practice"

Last revised: September 18, 2026