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Aperture mechanism of Canon 50mm f/1.8 II lens, with five bladesThe focal length value is not required when comparing two lenses of the same focal length; a value of 1 can be used instead, and the other factors can be dropped as well, leaving area proportion to the reciprocal square of the f-number N.

If two cameras of different format sizes and focal lengths have the same angle of view, and the same aperture area, they gather the same amount of light from the scene. In that case, the relative focal-plane illuminance, however, would depend only on the f-number N, so it is less in the camera with the larger format, longer focal length, and higher f-number. This assumes both lenses have identical transmissivity.

Though as early as 1933 Torkel Korling had invented and patented for the Graflex large format reflex camera an automatic aperture control,[13] not all early 35mm single lens reflex cameras had the feature. With a small aperture, this darkened the viewfinder, making viewing, focusing, and composition difficult.[14] Korling's design enabled full-aperture viewing for accurate focus, closing to the pre-selected aperture opening when the shutter was fired and simultaneously synchronising the firing of a flash unit. From 1956 SLR camera manufacturers separately developed automatic aperture control (the Miranda T 'Pressure Automatic Diaphragm', and other solutions on the Exakta Varex IIa and Praktica FX2) allowing viewing at the lens's maximum aperture, stopping the lens down to the working aperture at the moment of exposure, and returning the lens to maximum aperture afterward.[15] The first SLR cameras with internal ("through-the-lens" or "TTL") meters (e.g., the Pentax Spotmatic) required that the lens be stopped down to the working aperture when taking a meter reading. Subsequent models soon incorporated mechanical coupling between the lens and the camera body, indicating the working aperture to the camera for exposure while allowing the lens to be at its maximum aperture for composition and focusing;[15] this feature became known as open-aperture metering.

For some lenses, including a few long telephotos, lenses mounted on bellows, and perspective-control and tilt/shift lenses, the mechanical linkage was impractical,[15] and automatic aperture control was not provided. Many such lenses incorporated a feature known as a "preset" aperture,[15][16] which allows the lens to be set to working aperture and then quickly switched between working aperture and full aperture without looking at the aperture control. A typical operation might be to establish rough composition, set the working aperture for metering, return to full aperture for a final check of focus and composition, and focusing, and finally, return to working aperture just before exposure. Although slightly easier than stopped-

For some lenses, including a few long telephotos, lenses mounted on bellows, and perspective-control and tilt/shift lenses, the mechanical linkage was impractical,[15] and automatic aperture control was not provided. Many such lenses incorporated a feature known as a "preset" aperture,[15][16] which allows the lens to be set to working aperture and then quickly switched between working aperture and full aperture without looking at the aperture control. A typical operation might be to establish rough composition, set the working aperture for metering, return to full aperture for a final check of focus and composition, and focusing, and finally, return to working aperture just before exposure. Although slightly easier than stopped-down metering, operation is less convenient than automatic operation. Preset aperture controls have taken several forms; the most common has been the use of essentially two lens aperture rings, with one ring setting the aperture and the other serving as a limit stop when switching to working aperture. Examples of lenses with this type of preset aperture control are the Nikon PC Nikkor 28 mm f/3.5 and the SMC Pentax Shift 6×7 75 mm f/4.5. The Nikon PC Micro-Nikkor 85 mm f/2.8D lens incorporates a mechanical pushbutton that sets working aperture when pressed and restores full aperture when pressed a second time.

Canon EF lenses, introduced in 1987,[17] have electromagnetic diaphragms,[18] eliminating the need for a mechanical linkage between the camera and the lens, and allowing automatic aperture control with the Canon TS-E tilt/shift lenses. Nikon PC-E perspective-control lenses,[19] introduced in 2008, also have electromagnetic diaphragms,[20] a feature extended to their E-type range in 2013.

Optimal aperture depends both on optics (the depth of the scene versus diffraction), and on the performance of the lens.

Optically, as a lens is stopped down, the defocus blur at the Depth of Field (DOF) limits decreases but diffraction blur increases. The presence of these two opposing factors implies a point at which the combined blur spot is minimized (Gibson 1975, 64); at that point, the fOptically, as a lens is stopped down, the defocus blur at the Depth of Field (DOF) limits decreases but diffraction blur increases. The presence of these two opposing factors implies a point at which the combined blur spot is minimized (Gibson 1975, 64); at that point, the f-number is optimal for image sharpness, for this given depth of field[21] – a wider aperture (lower f-number) causes more defocus, while a narrower aperture (higher f-number) causes more diffraction.

As a matter of performance, lenses often do not perform optimally when fully opened, and thus generally have better sharpness when stopped down some – note that this is sharpness in the plane of critical focus, setting aside issues of depth of field. Beyond a certain point, there is no further sharpness benefit to stopping down, and the diffraction begins to become significant. There is accordingly a sweet spot, generally in the f/4 – f/8 range, depending on lens, where sharpness is optimal, though some lenses are designed to perform optimally when wide open. How significant this varies between lenses, and opinions differ on how much practical impact this has.

While optimal aperture can be determined mechanically, how much sharpness is required depends on how the image will be used – if the final image is viewed under normal conditions (e.g., an 8″×10″ image viewed at 10″), it may suffice to determine the f-number using criteria for minimum required sharpness, and there may be no practical benefit from further reducing the size of the blur spot. But this may not be true if the final image is viewed under more demanding conditions, e.g., a very large final image viewed at normal distance, or a portion of an image enlarged to normal size (Hansma 1996). Hansma also suggests that the final-image size may not be known when a photograph is taken, and obtaining the maximum practicable sharpness allows the decision to make a large final image to be made at a later time; see also critical sharpness.

In digital photography, the 35mm-equivalent aperture range is sometimes considered to be more important than the actual f-number. Equivalent aperture is the f-number adjusted to correspond to the f-number of the same size absolute aperture diameter on a lens with a 35mm equivalent focal length. Smaller equivalent f-numbers are expected to lead to higher image quality based on more total light from the subject, as well as lead to reduced depth of field. For example, a Sony Cyber-shot DSC-RX10 uses a 1" sensor, 24–200 mm with maximum aperture constant along the zoom range; f/2.8 has equivalent aperture range f/7.6, which is a lower equivalent f-number than some other f/2.8 cameras with smaller sensors.[22]

In scanning or sampling

The terms scanning aperture and sampling aperture are often used to refer to the opening through which an image is sampled, or scanned, for example in a

The terms scanning aperture and sampling aperture are often used to refer to the opening through which an image is sampled, or scanned, for example in a Drum scanner, an image sensor, or a television pickup apparatus. The sampling aperture can be a literal optical aperture, that is, a small opening in space, or it can be a time-domain aperture for sampling a signal waveform.

For example, film grain is quantified as graininess via a measurement of film density fluctuations as seen through a 0.048 mm sampling aperture.

See also[For example, film grain is quantified as graininess via a measurement of film density fluctuations as seen through a 0.048 mm sampling aperture.