Aperture

Aperture

noun | Camera & Optics

The adjustable opening within a camera lens that controls how much light passes through to the sensor or film plane. A wider aperture admits more light; a narrower aperture admits less. Aperture is expressed as an f-stop (f/) or T-stop (T/) number: a lower number indicates a wider opening, a higher number a narrower one. Aperture is one of the three primary exposure controls alongside shutter speed and ISO, and it is the principal control for depth of field.

Quick Reference

The Explanation: How & Why

The aperture is formed by a set of overlapping metal blades inside the lens barrel called the iris diaphragm. When the iris opens, the blades retract to create a large circular opening; when it closes, the blades advance to create a small opening. The size of this opening directly controls the volume of light reaching the sensor per unit of time.

The f-stop scale is logarithmic: each full stop doubles or halves the amount of light. The standard full-stop sequence is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Moving from f/2.8 to f/2 doubles the light (one stop brighter); moving from f/2.8 to f/4 halves it (one stop darker). Cinema lenses are typically marked in T-stops rather than f-stops. T-stops measure the actual light transmission through the optical elements, accounting for the glass absorption and coating losses that the theoretical f-stop calculation ignores. T-stops are more accurate for exposure matching between different lenses.

Aperture has a second consequence as important as exposure: depth of field. A wide aperture (low T or f number) produces a shallow depth of field -- only a narrow band of the scene is in sharp focus, and objects in front of and behind the focus point fall progressively out of focus. A narrow aperture (high T or f number) produces a deep depth of field -- more of the scene from foreground to background is in acceptable focus. This relationship between aperture and depth of field is one of the most fundamental creative parameters in cinematography.

Wide apertures are used for aesthetic separation: isolating a subject from a blurred background, creating the characteristic "bokeh" quality of out-of-focus areas. Narrow apertures are used for deep focus work: keeping a wide range of depth in simultaneous sharp focus for environmental or compositional reasons. The choice of aperture is therefore both an exposure decision and a compositional and emotional one.

Neutral density (ND) filters are used to decouple aperture from exposure. On a bright exterior day, the light level may be so high that correct exposure would require stopping down to f/11 or f/16 -- eliminating the shallow depth of field the DP wants. Adding ND filtration reduces the light entering the lens, allowing the DP to shoot at a wide aperture (T2 or T2.8) for shallow depth while still achieving correct exposure.

Historical Context & Origin

The iris diaphragm as an adjustable aperture control has been part of camera lens design since the mid-19th century. The f-stop nomenclature was standardised internationally in the early 20th century. The development of wide-aperture cinema lenses -- the Zeiss Super Speed series (T1.3), the Cooke S4/i series, and later the Master Prime and ARRI/Zeiss Ultra Prime lines -- gave cinematographers progressively more control over depth of field as a creative variable. The extreme shallow depth of field work of Gordon Willis on The Godfather (1972), shot at very wide apertures in low light to create a dark, compressed visual world, became one of the most influential cinematographic choices of the 1970s. Willis's refusal to compromise his aperture choice to make the exposure "safe" for the studio established the primacy of the DP's creative aperture decision in professional cinematography.

How It's Used in Practice

Scenario 1 -- Narrative Drama (DP / 1st AC): The DP sets a 85mm prime at T1.4 for a close-up in a candlelit interior. The shallow depth of field at this aperture means the focus plane is approximately 2 inches deep at the 4-foot subject distance. The 1st AC confirms the follow focus marks and acknowledges the challenge: the actor must hit their mark within an inch for the eyes to remain in focus. The DP accepts this constraint because the visual quality of T1.4 in that light is irreplaceable.

Scenario 2 -- Exterior Day (DP / 1st AC): On a bright exterior, the DP wants to shoot at T2 for shallow depth of field in a medium shot. Without filtration, the correct exposure at ISO 800 and 1/48 shutter would require T11. The 1st AC fits a 6-stop IRND filter (variable neutral density) to bring the stop back to T2. The DP confirms exposure on the waveform.

Scenario 3 -- Deep Focus (DP): For a compositional shot in which a character in the foreground and a critical object in the far background must both be sharp, the DP stops down to T8. The depth of field calculator confirms that at 40mm on a Super 35 sensor at T8, the depth of field at 6 feet extends from approximately 3.5 feet to infinity -- both foreground and background are in focus. The DP raises the ISO and adds a Kino-Flo unit to compensate for the stopped-down exposure.

Usage Examples in Sentences

"Open up to T1.4 for the close-up and make sure the AC has pull marks at the eyeline."

"At T8, the depth of field covers the whole room -- both the detective and the body in the background stay sharp."

"The ND filter lets us shoot at T2 in full sun without blowing the exposure."

"T-stop and f-stop measure the same thing conceptually, but T-stop is the accurate real-world measurement -- always use T-stops when matching lenses on a cinema set."

Common Confusions & Misuse

F-Stop vs. T-Stop: An f-stop is a mathematically derived ratio of focal length to aperture diameter -- it does not account for light loss through the glass elements of the lens. A T-stop is a measured value of actual light transmission. Two lenses set to f/2.8 may transmit noticeably different amounts of light due to differences in glass count and coating. For cinema work where exposure consistency between lens changes matters, T-stops are the appropriate unit. F-stops appear on photographic lenses and are perfectly adequate for single-lens work where absolute consistency is not critical.

Aperture vs. Iris: The iris is the mechanical assembly of blades inside the lens that creates the aperture. The aperture is the opening the iris creates. In casual production conversation, "iris," "aperture," and "stop" are used interchangeably ("open the iris," "widen the aperture," "go to T2"). All three refer to the same adjustable opening. The precision distinction matters in technical discussions about lens design, not in on-set direction.

Variations by Context

Related Terms

• Depth of Field -- Directly controlled by aperture; the primary creative consequence of aperture choice
• Shutter Speed -- The second exposure control; adjusted alongside aperture to achieve correct exposure
• ISO -- The third exposure control; raises or lowers sensor sensitivity to complement aperture setting
• Lens -- The optical instrument containing the aperture iris
• T-Stop -- The cinema standard for aperture measurement; more accurate than f-stop for multi-lens productions

See Also / Tools

The Depth of Field Calculator shows the precise focus range at any aperture, focal length, and subject distance. The Exposure / Shutter / Focal Length Calculator connects aperture to the other exposure variables to achieve correct brightness at the desired stop.