Aperture (optics)

Large (1) and small (2) apertures.

Aperture ( Latin apertura - hole) in optics is a characteristic of an optical device that describes its ability to collect light and withstand diffraction blurring of image details. Depending on the type of optical system, this characteristic may be linear or angular . As a rule, the so-called aperture diaphragm is especially distinguished among the details of an optical device, which most severely limits the diameters of the light beams passing through the optical instrument. Often the role of such an aperture diaphragm is played by the frame or edges of one of the optical elements ( lenses , mirrors , prisms ).

An input aperture is a characteristic of the ability of an optical system to collect light from an object of observation . If the object is distant (like a telescope or a conventional photo lens), then the aperture is measured in a linear form - as the diameter of the light beam at the entrance to the optical system, which is limited by the aperture diaphragm and reaches the image. In telescopes, this diameter is usually equal to the diameter of the first optical element in the direction of light (lens or mirror). In photographic lenses (especially wide-angle ones), the size of the first lens is usually much larger than the input aperture and its size should already be calculated. The input aperture of the lens is equal to the product of its focal length f 'by the relative aperture or the quotient of the focal length by the aperture value. If the object of observation is close (like a magnifying glass, a microscope lens or a projector), then the aperture is measured in angular form - as the angle of a light beam emanating from the points of the object of observation and entering the optical system.

The output aperture is a characteristic of the ability of an optical system to collect light in an image . If the image is remote (like a telescope , magnifier or projector ), then the aperture is measured in a linear form - this is the diameter of the light beam at the exit of the optical system, in the area of ​​the so-called exit pupil . If the image is close (like a photo lens ), then the aperture is characterized by the angle of convergence of the light beams.

Aperture angle - the angle between the extreme beam of a conical light beam at the entrance (exit) of the optical system and its optical axis.

Angular aperture - the angle between the extreme rays of a conical light beam at the input (output) of the optical system.

Numerical aperture

• in fiber optical systems , the sine of the maximum angle between the axis and the beam, for which the conditions of total internal reflection are satisfied during the propagation of optical radiation through the fiber. It characterizes the efficiency of introducing light rays into an optical fiber and depends on the design of the fiber.
• in light microscopy is equal to the product of the refractive index of the medium between the object and the lens by the sine of the aperture angle. It is this value that most fully determines simultaneously the aperture, the resolution of the microscope objective. To increase the numerical aperture of the lenses in microscopy, the space between the lens and the coverslip is filled with immersion liquid.

The lens aperture is the diameter D of the light beam at the entrance to the lens and entirely passing through its aperture diaphragm. This value also determines the diffraction limit of the resolution of the lens. To estimate the resolution in angular seconds , the formula 140 / D is used, where D is the lens aperture in millimeters .