field of view
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field of view(field) The area made visible by the optical system of an instrument such as a telescope at a particular setting. It is expressed in the form of its angular diameter. The field of view of a telescope usually increases with decreasing magnification, and depends on the eyepiece in use. A wide field may be obtained with specially designed eyepieces or with telescopes such as the Schmidt. If the telescope optics produce a flat field, the image will be focused at the center as well as at the edge of the field of view.
Field of View
The field of view of an optical system is the part of space or of a plane imaged by the system. The size of the field is determined by the system’s components—such as stops or the mounts of lenses, prisms, and mirrors—that restrict the beam of light rays. A distinction is made between angular and linear fields of view. The former is measured in angular units and is used in dealing with systems designed for observing very distant—essentially infinitely remote—objects; telescopes, field glasses, and many types of cameras are examples of such systems. The linear field of view is measured in millimeters or centimeters and is used with systems, such as microscopes, in which the distance to the object is small.
In Figure 1, A is the center of the system’s entrance pupil. The angular field of view is the angle 2ω at which the entrance port S1SS2 and the corresponding part of the object plane O1O2 can be seen from A. The linear field of view is the linear dimension O1O2. OO is the system’s axis of symmetry. In the general case, the planes O1O2 and S1S2 are not coincident, and vignetting with a ring width of BB1 occurs. If, however, the plane S1S2 is made coincident with the object plane, the boundary of the field of view is sharp. Efforts are made to achieve this effect in many optic tubes and other optical instruments by placing a field stop in the focal plane of the objective.
The angular field of view in the object space is inversely proportional to the angular magnification of the system. In binoculars the field ranges from 5° to 10°, and in the largest telescopes it does not exceed a few minutes of arc. In special wide-angle photographic lenses it can reach 120° to 140° or even 180°. The vast majority of microscopes are fitted with a set of replaceable eyepieces whose magnifications and, consequently, linear fields of view 2l in the object space are different. Eyepieces with 2l = 18 mm are often used. In many eyepieces, however, the field is greater or smaller than this value. In polarizing microscopes and stereomicroscopes, eyepieces with a field of up to 25 mm (wide-angle eyepieces) are often used. The linear field of view of a microscope as a whole is equal to 2l/β, where β is the linear magnification of the microscope objective.