Editing Aperture (section)

==Application==
[[File:Yerkes Observatory Astro4p6.jpg|thumb|Alvin Clark polishes the big [[Yerkes Observatory]] [[Great Refractor]] objective lens, with 40 inches 102&nbsp;cm across, in 1896.]]
The aperture stop is an important element in most optical designs. Its most obvious feature is that it limits the amount of light that can reach the image/[[film plane]]. This can be either unavoidable due to the practical limit of the aperture stop size, or deliberate to prevent saturation of a detector or overexposure of film. In both cases, the size of the aperture stop determines the amount of light admitted by an optical system. The aperture stop also affects other optical system properties:
*The opening size of the stop is one factor that affects DOF ([[depth of field]]). A smaller stop (larger [[f number]]) produces a longer DOF because it only allows a smaller angle of the cone of light reaching the image plane so the spread of the image of an object point is reduced. A longer DOF allows objects at a wide range of distances from the viewer to all be in focus at the same time.
*The stop limits the effect of optical [[Aberration in optical systems|aberrations]] by limiting light such that the light does not reach edges of optics where aberrations are usually stronger than the optics centers. If the opening of the stop (called the ''aperture'') is too large, then the image will be distorted by stronger aberrations. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger aperture and therefore greater light collecting ability.
*The stop determines whether the image will be [[vignetting|vignetted]]. Larger stops can cause the light intensity reaching the film or detector to fall off toward the edges of the picture, especially when, for off-axis points, a different stop becomes the aperture stop by virtue of cutting off more light than did the stop that was the aperture stop on the optic axis.
*The stop location determines the [[Telecentric lens|telecentricity]]. If the aperture stop of a lens is located at the front focal plane of the lens, then it becomes image-space telecentricity, i.e., the lateral size of the image is insensitive to the image plane location. If the stop is at the back focal plane of the lens, then it becomes object-space telecentricity where the image size is insensitive to the object plane location. The telecentricity helps precise two-dimensional measurements because measurement systems with the telecentricity are insensitive to axial position errors of samples or the sensor.

In addition to an aperture stop, a photographic lens may have one or more ''field stops'', which limit the system's [[Angle of view|field of view]]. When the field of view is limited by a field stop in the lens (rather than at the film or sensor) [[vignetting]] results; this is only a problem if the resulting field of view is less than was desired.

In astronomy, the opening diameter of the aperture stop (called the ''aperture'') is a critical parameter in the design of a [[telescope]]. Generally, one would want the ''aperture'' to be as large as possible, to collect the maximum amount of light from the distant objects being imaged. The size of the aperture is limited, however, in practice by considerations of its manufacturing cost and time and its weight, as well as prevention of aberrations (as mentioned above).

Apertures are also used in laser energy control, close aperture [[z-scan technique]], diffractions/patterns, and beam cleaning.<ref>{{cite journal|last1=Rashidian Vaziri|first1=M R|title=Role of the aperture in Z-scan experiments: A parametric study|journal=Chinese Physics B|volume=24|issue=11|pages=114206|doi=10.1088/1674-1056/24/11/114206|bibcode = 2015ChPhB..24k4206R |year=2015|s2cid=250753283 }}</ref> Laser applications include [[spatial filter]]s, [[Q-switching]], high intensity x-ray control.

In light microscopy, the word aperture may be used with reference to either the [[Condenser (microscope)|condenser]] (that changes the angle of light onto the specimen field), field iris (that changes the area of illumination on specimens) or possibly objective lens (forms primary images). ''See'' [[Optical microscope]].