Objective

OBJECTIVE, a system of lenses serving to produce a real image of an object (in photographic cameras, projection lanterns, microscopes, telescopes, etc.). The main characteristics of an objective are its focal length and aperture i, equal to the ratio of the area of the D active (undiaphragmed) aperture O to its focal length, i.e., i=D/F. In most modern objectives, diaphragms are designated directly in units of aperture (exceptions are the Voigtländer system, where the unit of aperture is taken as the aperture with an opening of F:3.16, and the old Zeiss system, where the numbering unit is taken as the diaphragm F:50, and the remaining diaphragms are designated by a series of numbers corresponding to the squares of the apertures; in Goerz photographic objectives, diaphragms are sometimes designated by numbers proportional to the exposure time). The field of view (image) of an objective is characterized by the so-called image (vision) angle or the angle of the light cone. This angle is determined graphically: when the objective is focused on infinity, an image in the form of a bright circle is obtained in the focal plane, and the diameter of the latter is measured. An isosceles triangle is constructed on paper, in which the base AB is equal to the diameter of the bright circle, and the height ZF is the focal length of the objective. The angle α = ∠AZB is measured with a protractor, which will be the angle of view. This angle depends not only on the focal length of the objective, but also on the compactness of its design (Figure 1). Thus, anastigmats have a larger field of view than aplanats with the same focal length. Modern wide-angle objectives make it possible to capture an angle of up to 135° from a close distance. An ordinary converging lens can serve as the simplest objective. Such an objective gives a very imperfect and unsharp image because it possesses all the flaws inherent in thick optical glasses. In artistic photography, where it is often precisely required to avoid image sharpness and, conversely, to "smudge" it,

Figure 1. Image angle.

Figure 2. Image angle of 110°.

Figure 3. Image angle of 75°.

to impart "softness" (for example, in portrait photography), a simple spectacle-glass monocle (D=25, F=20 cm, d~5 cm) is used as an objective. A simple single-lens objective is also found in cheap projection apparatuses. More perfect are objectives corrected (compensating) for chromatic aberration—achromats (see Achromatism, achromats) (the first achromat was built by Chevalier in 1830; in photographic equipment, achromats are known under the name of "landscape lens") and for spherical aberration—aplanats (the first was built by Voigtländer in 1840; see Aplanat). Next come apochromats (see), in which both chromatic and spherical aberrations are corrected. The most perfect objectives are anastigmats (the first was constructed by Rudolph at C. Zeiss in 1830; the best are the "Tessar" of Zeiss (Figs. 2 and 3), the "Dagor" of Goerz, the "Collinear", "Heliar", and "Dynaar" of Voigtländer), which are corrected in addition to aberrations for astigmatism and for the curvature of lines and planes. Such objectives give exceptionally perfect images and are the highest achievement of modern optical technology. All these objectives have found

Figure 4. Ermanox camera with Ernostar objective.

especially wide application in photographic and projection equipment. Recently, for the possibility of photo- and cinematography in weak lighting, particularly high-aperture objectives have become widespread. Ernemann has produced objectives with an aperture up to F:1.8 (the "Ermanox" camera with the "Ernostar" objective—Fig. 4). In such devices, the objectives turn out to be larger in size than the camera itself. In particularly high-aperture objectives, the correction of flaws is less achievable than in ordinary ones. In telescopes, achromatic and apochromatic objectives are used. The highest perfection has been achieved in microscopic objectives. (See Microscope).