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objective(ŏb-jek -tiv) (object lens; object glass) The lens or lens system in a refracting telescope that faces the observed object. The focal plane in which the image forms, in the absence of other components, is termed the prime focus.See also aperture.
the part of an optical system that faces an object; also, an independent optical system that forms a real optical image of an object. The image is viewed visually through an eyepiece or is produced on a flat or, less often, curved surface (a photographic emulsion, the photocathode of a television camera tube or image converter, ground glass, or a screen).
Objectives may be divided into three classes according to design: (1) lens (dioptric) objectives (refractors), which are the most common; (2) mirror (catoptric) objectives (catoptric reflectors); and (3) mirror-lens (catadioptric) objectives. A distinction is made among the objectives of field glasses or telescopes, which produce a reduced image; microscope objectives, which produce a magnified image; and photographic and projection objectives, which produce a reduced or magnified image, depending on the design and method of use.
The most important optical characteristics of objectives are the focal length, which determines the optical magnification of the objective for a given distance of an object from the objective; the diameter of the entrance pupil; the relative aperture and the transmission, which is expressed in terms of the relative aperture; and the field of view. The quality of the image formed by the objective depends on the resolving power of the objective, the contrast transmission factor, the coefficients of total and spectral light transmission, the light scattering coefficient, and the falling-off of the illumination over the image field.
Objectives of field glasses and telescopes. The distance to objects reproduced by the objectives of field glasses and telescopes is assumed to be very large (virtually infinitely large). Therefore, the objectives are characterized by angular rather than linear dimensions. Correspondingly, objectives of this group are characterized by the angular magnification γ, the angular resolving power a, and the field of view 2ω = 2ω′/γ, where 2ω′ is the field of view of the part of the optical system that is behind the objective (usually the eyepiece). In turn, γ= f1/f1, where f1 is the focal length of the objective and f2 is the first focal length of the subsequent part of the system. The resolving power of an objective is determined (in seconds of arc) by the formula α″ = 120″/D, where D is the diameter of the entrance pupil of the objective, expressed in millimeters (most often the mounting of the objective is the entrance pupil). The image brightness (transmission of the objective) is proportional to the square of the relative aperture, (D/f1)2.
The objectives of field glasses used for measurement and observation, and also of geodetic instruments, have entrance pupils a few centimeters in diameter. The narrow field of view of most field glasses (no more than 10°–15°, and usually less) makes possible the use of objectives of comparatively simple design; the lens objectives generally consist of two cemented lenses and are corrected only for spherical and chromatic aberrations. Objectives consisting of three or more lenses, in which coma and some other aberrations of optical systems are corrected, are less widely used. By the 1970’s, Maksutov meniscus systems were being used in geodetic instruments. The relative apertures of the objectives of observation glasses and geodetic instruments vary over a wide range (from about 1:20 to 1:5).
The diameters of lens and mirror-lens telescope objectives range from about 0.5 to 1.0 m (the maximum is D = 1.4 m). Two-lens objectives, which are also corrected only for spherical and chromatic aberrations, are used in refractors. Three-lens and four-lens objectives are used in astrographs designed for photography of the stellar sky; they are usually corrected for all aberrations except curvature of field. The field of view of astrograph objectives reaches 6°; in two-lens objectives of refractors it usually decreases with increasing lens diameter (for the largest lenses it is less than 1°). The relative apertures of large refractors are of the order of 1:20 to 1:10; in astrographs they are larger (1:1.4 to 1:1.2). In Schmidt telescopes and Maksutov meniscus systems the field of view reaches 5° at a relative aperture of about 1:3. The largest objective of a mirror telescope has D = 5 m (a reflector with a parabolic mirror in the Hale Observatory on Mount Palomar in the USA); in the USSR a reflector with a parabolic mirror about 6 m in diameter is under construction. The field of view of such objectives does not exceed a few minutes of arc; in the objectives of telescopes based on the Ritchey-Chrétien reflector system with a hyperbolic primary mirror, the field of view may be as much as 1”. The aberrations of such objectives (except for chromatic and spherical aberrations) are considerable and are corrected by introducing additional (correction) lenses and mirrors, called compensators. In the objectives of modern large reflectors it is possible to interchange the auxiliary mirrors, thus permitting work with relative apertures of about 1:4, 1:10, and 1:30.
The objectives used in satellite tracking systems (called satellite cameras) and for photographing bodies (such as meteors) moving in the upper layers of the atmosphere are also classified as astronomical objectives. Their characteristics are close to those of astrographs on the one hand and to certain types of photographic objectives on the other. They are corrected for all aberrations except curvature of field, their field of view may reach 30°, and their relative apertures are usually large (up to 1:1.2). A typical example is the Astrodar objective of a satellite camera, based on the Maksutov system. It is distinctive in that all its refracting and reflecting surfaces are spherical and at the same time concentric. The effective diameter of this objective is 50 cm, and f ≈ 70 cm (therefore, the relative aperture is 1:1.4); the field of view is 5° × 30°.
Photographic objectives. Photographic objectives, including objectives used in cinematography and reproduction, differ from the objectives of the previous group in that the images they produce must be sharp all the way to the edge of the photographic film or other receiver, whose dimensions may be comparatively large. Therefore, the field of view of a sharp image is much larger in such objectives than in those of field glasses (more than 50°). To achieve sharpness and high contrast of an undistorted plane image at large angles of the field of view, all primary aberrations (spherical and chromatic aberrations, coma, astigmatism, distortion, and curvature of field)—and in many cases, the most significant aberrations of higher order—must be carefully corrected. This leads to considerable complication in the design, which increases with the relative aperture and field of view (the number of lenses and mirrors increases, and/or their shape becomes more complex). Figure 1 shows several diagrams of the most common photographic lens objectives. Objectives constructed according to the same optical design may have different optical characteristics (focal length, relative aperture, and field of view) and may be used for various purposes.
A distinction is made among photographic objectives used in amateur and professional photography and cinematography and those used in photographic reproduction, television, aerial photography, fluorography, and astrography, as well as objectives for invisible regions of the spectrum (infrared and ultraviolet). The objectives of a given group may be normal (or universal), high-power, wide-angle, or telescopic. Normal (universal) objectives are the most common type. They are usually anastigmats and produce a sharp plane image with a moderately large relative aperture and field of view. Their focal lengths range from 40 to 150 mm; relative apertures, 1:1.8 to 1:4; and average field of view, about 50°. High-power objectives with relative apertures from 1:1.8 to 1:0.9 (in some designs, particularly mirror-lens designs, up to 1:0.8) are used for photography at low light levels; their field of view is usually less than that of universal objectives. Wide-angle objectives have a field of view greater than 60°, reaching 180° in some lenses (for example, the Hill objective shown in Figure 1 has a field of view of 180°, with a relative aperture of 1:22). Such objectives play a particularly important role in aerial photography. The focal lengths of wide-angle objectives are usually 100–500 mm; their relative apertures are characterized by medium and low values (1:5.6 or less). They are difficult to correct for such aberrations as distortion, curvature of field, and astigmatism. Objectives with corrected distortion are called orthoscopic. The distortion of objectives with a field of view approaching 180° (from about 120° to 180°) is not corrected; it can be partially corrected by using a special objective in the printing process. Significant distortions of perspective are characteristic of the images formed by such objectives (called distorting objectives). These objectives are used, for example, to produce special compositions in the photography of architectural ensembles and landscapes. The larger the field of view, the more noticeably the image brightness decreases toward the edge (in proportion to the cosine of the fourth power of half the angle of field of view). In objectives for amateur and professional photography the nonuniformity of brightness is corrected in calculating the aberrations of the objective; in other types of photographic objectives brightness is equalized by means of special filters.
Objectives whose focal length exceeds three times the value of the linear field of view are classified as telescopic objectives (for most photographic objectives it is 100–2,000 mm). Telescopic objectives are used to photograph distant objects on a large scale. Their field of view is usually less than 30°, and their relative aperture does not exceed 1:4.5 to 1:5.6.
The provision of identical quality of correction of all aberrations of photographic objectives is extremely difficult, particularly in high-speed, wide-angle, and special objectives. Therefore, compromise solutions are found by changing the requirements for aberration correction relative to the function of the objective. For example, field aberrations are less thoroughly corrected in high-power photographic objectives, but the field of view is reduced as a result. In the case of objectives with long focal lengths, special measures are taken to correct chromatic aberrations.
The selection of illumination in the image plane of a photographic objective depends on the brightness of the object, the sensitivity of the photographic material or other light receiver, and the required depth of the image space (depth of focus). A change in illumination is brought about by changing the relative aperture of the objective by means of a stop of variable diameter, such as an iris diaphragm. The mounting of the objective carries a scale from which the required relative aperture can be set off (the maximum value of the aperture is usually indicated in the description of the objective). The illumination of the image plane is proportional to the square of the ratio of the diameter of the entrance pupil of the objective to its focal length—called the geometric light power of the objective. The physical light power of the objective is found by multiplying this quantity by a factor determined by the energy losses of the light during its transmission through the objective (losses to absorption in the glass and to reflection from optical surfaces). To increase the physical light power (that is, to reduce light losses), modern photographic objectives are coated. The selection of special single-layer and multilayer coatings makes possible not only an increase in the total transmission of an objective but also balancing of its spectral transmission with the spectral sensitivity of the three layers of color-reversing film. This ensures proper reproduction of the colors of objects depicted on such film.
Pancratic objectives with variable focal length are widely used (many motion-picture objectives are of this type). The focal length is varied by moving certain components of the objective; the relative aperture usually remains unchanged. In particular, with such objectives it is possible to change the image scale without changing the position of the object or the image plane (when the components of an objective are moved and its focal length changed, the positions of the principal planes of the objective change). According to optical-correction properties, objectives with a variable focal length are divided into two groups: (1) variable-focus lenses, whose optical design is corrected as a whole for all aberrations, and (2) zoom lenses, which are systems consisting of an objective and a front-mounted afocal attachment whose aberrations are corrected separately. High-quality images are produced in pancratic objectives by increasing the number of lenses and components. Such objectives are complex systems that consist of 11–20 lenses.
Projection objectives are of the same type as photographic objectives; in principle, they differ only in the direction of travel of the light rays. A distinction is made between objectives for slide projection in transmitted light and objectives for episcope projection in reflected light. Reproduction objectives, which are used to produce images of such things as flat objects, blueprints, and maps, make up a special subgroup that is classified with the photographic objectives. Projection objectives, reproduction objectives, and photographic objectives used at short distances from the object are characterized not by the angular magnification but by the linear magnification (the scale of the image in the proper sense) and by the linear dimensions of the field of view and the numerical aperture. In this regard they are similar to microscope objectives.
Microscope objectives. Microscope objectives are distinguished by their placement in direct proximity to the object. Their focal lengths are short (from 30–40 mm to 2 mm). Among the main optical characteristics of microscope objectives are the numerical aperture A, which is equal to n1 sin u1 (where n1 is the index of refraction of the medium in which the object is located and u1 is the semivertex angle of the light cone incident on the objective from a point on the object lying on the optical axis of the objective); the linear magnification β; the linear dimensions 2l of the field of view sharply reproduced by the objective; and the distance from the object plane to the image plane. The quantity A determines both the illumination of the image, which is directly proportional to A2, and the linear limit of resolution of the microscope (that is, the shortest discernible distance on the object), which for self-luminous objects (assuming no aberration) is ε = 0.51 λ/A, where λ is the wavelength of the light. If the object is located in air (n = 1, a “dry” objective), then A cannot exceed 1 (actually 0.9). By placing the object in a strongly refractive liquid (n > 1), called an immersion objective, touching the surface of the first lens of the objective, A can be increased to 1.4–1.6. In modern microscopes, β reaches 90–100 ×; the total magnification of a microscope is Γ = βΓ′, where Γ′ is the angular magnification of the eyepiece. The linear field 2l is related to the diameter D of the diaphragm of the field of view of the eyepiece by the relation 2l = D/β. As A and β increase, the objective becomes more complex, since the requirements for image quality are very great: the resolving power of an objective must be virtually identical to the value given above for an ideal (nonaberrational) objective. The designs of the most advanced microscope objectives—the “planachromats” and “planapochromats”—satisfy this condition. A diagram of one of the best Soviet-made planapochromats is shown in Figure 2.
The objectives of spectral instruments (which are close to photographic objectives in many properties), and also special objectives designed for use with lasers, make up special groups.
REFERENCESTudorovskii, A. I. Teoriia opticheskikh priborov, 2nd ed., parts 1–2. Moscow-Leningrad, 1948–52.
Sliusarev, G. G. Metody rascheta opticheskikh sistem, 2nd ed. Leningrad, 1969.
Flügge, J. Das photographische Objektiv. Wiesbaden, 1955.
Rusinov, M. M. Fotogrammetricheskaia optika. Moscow, 1962.
Mikroskopy. Edited by N. I. Poliakov. Moscow, 1969.
Michel, K. Osnovy teorii mikroskopa. Moscow, 1955. (Translated from German.)