Aperture - Application

Application

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, as in a telescope where one wants to collect as much light as possible; or deliberate, to prevent saturation of a detector or overexposure of film. In both cases, the size of the aperture stop is constrained by things other than the amount of light admitted; however:

• The size of the stop is one factor that affects depth of field. Smaller stops (larger f numbers) produce a longer depth of field, allowing objects at a wide range of distances to all be in focus at the same time.
• The stop limits the effect of optical aberrations. If the stop is too large, the image will be distorted. More sophisticated optical system designs can mitigate the effect of aberrations, allowing a larger stop and therefore greater light collecting ability.
• The stop determines whether the image will be vignetted. Larger stops can cause the 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.
• A larger aperture stop requires larger diameter optics, which are heavier and more expensive.

In addition to an aperture stop, a photographic lens may have one or more field stops, which limit the system's 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.

The biological pupil of the eye is its aperture in optics nomenclature; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil in optics parlance) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4 mm in diameter, although it can range from 2 mm (f/8.3) in a brightly lit place to 8 mm (f/2.1) in the dark.

In astronomy, the 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 cost and weight, as well as prevention of aberrations (as mentioned above).

Apertures are also used in laser energy control, focusing, diffractions/patterns, and beam cleaning. Laser applications include spatial filters, Q-switching, high intensity x-ray control.

In light microscopy, the word aperture may be used with reference to either the condenser (changes angle of light onto specimen field), field iris (changes area of illumination) or possibly objective lens (forms primary image). See Optical microscope.