Page:EB1911 - Volume 21.djvu/533

APPARATUS]  Photographic: Objectives or Lenses.

The objective is the most important item of photographic apparatus, because upon it depends the perfection with which a correct and well-defined picture is projected upon the plane surface of the sensitive plate of objects in the different planes forming the field of view, which naturally would come to a focus on a series of curved surfaces. This flattened picture must be equally illuminated and sharply defined, within a limit of confusion from to of an inch, over a sufficiently wide angle. A good objective must also pass sufficient light to produce the required effect on the photographic plate with short exposures; the chemical and visual foci must coincide exactly, and it must not distort straight or parallel lines. The fulfilment of these conditions is complicated by the presence of sundry focal displacements or aberrations. (1) Spherical aberration, or non-coincidence of the foci of the central and marginal pencils of rays passing through the lens. It is corrected by varying the curves of the component lenses and by the use of a diaphragm. (2) Coma, or blur, due to lateral spherical aberration of oblique rays, and mostly found in unsymmetrical combinations and single view lenses. It is partly eliminated by the diaphragm. (3) Astigmatism, which accompanies coma in single lenses, and is usually present in symmetrical aplanats, manifests itself by forming two sets of images of points off the axis, lying in two separate curved surfaces, one set focusing tangentially as more or less horizontal lines, the other radially as more or less vertical lines. It increases with the obliquity of the rays and causes want of definition and difference of focus between horizontal and vertical lines away from the centre. (4) Curvature of field, also increasing with the obliquity of the rays. (5) Distortion, outward or inward, according to the nature and construction of the objective. With the single meniscus view lens, used with its concave surface towards the object and a diaphragm in front, a square will appear barrel shaped from inward contraction of the lines towards the centre; but with the convex surface towards the object and the diaphragm behind, it will appear with concave sides from outward expansion from the centre. It can be corrected by using two such lenses with the convex sides outwards and a central diaphragm, as in periscopic or rectilinear lenses. Lenses of the orthoscopic and telephoto types generally show the latter form of distortion. (6) Chromatic aberration, produced by the dispersion of the white light passing through the lens, and the different coloured rays composing it coming to a focus at different distances from the visual focus in the order of their wave-lengths. It thus affects both the positions and sizes of the image for the different colours. For ordinary photographic work it suffices for the blue-violet and yellow rays to be coincident, but for the new processes of photography in three colours, apochromatic lenses, in which perfect coincidence of the coloured rays is secured, are required to obtain the accurate register of the three images. The corrections are effected by compensating lenses of different refractive powers (see ).

In constructing photographic objectives these aberrations and distortions have to be neutralized, by regulating the curves of the different positive and negative component lenses, the refractive and dispersive indices of the glasses from which they are made, and the distances of the refracting surfaces, so as to make the objective as far as possible stigmatic or focusing to a point, giving an image well defined and undistorted. This perfect correction could newer be effected in objectives made before 1887, and very few could be effectively used at their full apertures, because although linear distortion could be overcome there were always residual aberrations affecting the oblique rays and necessitating the use of a diaphragm, which by lengthening out the rays caused them to define clearly over a larger surface, at the expense of luminous intensity and rapidity of working. The introduction of rapid gelatin dry plates enabled photographs to be taken with much greater rapidity than before, an led to a demand for greater intensity of illumination and better definition in lenses to meet the requirements of the necessarily very rapid exposures in hand cameras. For studio and copying work quick-acting lenses are also valuable in dull weather or in winter.

The rapidity of a lens with a light of given intensity depends upon the diameter of its aperture, or that of the diaphragm used, relatively to the focal length. In order, therefore, to obtain increased rapidity combined with perfect definition, some means had to be found of constructing photographic objectives with larger effective apertures. This necessity had long been recognized and met by many of the best makers for objectives of the single meniscus and aplanatic types, but with only partial success, because such objectives are dependent upon the diaphragm for the further correction necessary to obtain good definition over an extended field. The difficulty was in the removal of astigmatism and curvature of the field, which, as J. Petzval had shown, was impossible with the old optical flint and crown glasses. In 1886 Messrs E. Abbe and O. Schott, of Jena, introduced several new varieties of optical glasses, among them new crown glasses which, with a lower dispersion than flint glass, have a higher instead of a lower refractive power. It was thus rendered possible to overcome the old difficulties and to revolutionize photographic optics by

enabling objectives to be made free from astigmatism, working at their full apertures with great flatness of field independently of distortion, the diaphragm, which is now chiefly used to extend the area of definition or angle of view, and the so-called “depth of focus” for objects in different planes. They are also sometimes classified according to their rapidity, as expressed by their effective apertures, into “extra rapid,” with apertures larger than f&#8202;/6; “rapid,” with apertures from f&#8202;/6 to f&#8202;/8; “slow,” with apertures less than f&#8202;/11. Another classification is according to the angle of view, “narrow angle” up to 35°; “ medium angle” up to 60°; “wide angle” up to 90°, 100°, or more. Many lenses are made in series, differing in rapidity and angle of view as well as in length of focus.

1. Single Achromatic Combination or Landscape Lens.—This is the earliest form of (photographic objective, evolved from W. H. Wollaston’s improve singe periscopic meniscus camera obscura lens (1812). It was made achromatic by Ch. Chevalier, and so by L. J. M. Daguerre, though it required correction for chemical focus, as did the object glasses of telescopes or opera glasses first used for photography. The single landscape lens usually consists of an achromatic compound meniscus, formed of a biconvex positive crown cemented to a biconcave negative flint to secure achromatism and partially correct the spherical aberration, and may be taken

. 14.—Single Landscape Lens. as the type of the “old photographic achromat” (fig. 14). It is used with its concave side towards the object and a diaphragm in front, thus producing inward or barrel-shaped distortion inherent in this type of objective, and rendering it unsuitable for copying or architecture, though not very noticeable in landscape work. The full aperture has to be largely reduced by a diaphragm to improve definition; so it is slow, though many improved forms have been brought out. It has always been popular for pure landscape work on account of the equality of illumination over the plate, depth of focus, and the softness and brilliancy of the image owing to its thinness and freedom from reflecting surfaces. In some of its improved and “long focus” forms it is preferred by portraitists for large heads, on account of the general softness it gives when used with large apertures.

The following are some of the best-known improved objectives of this type: T. Grubb’s “Aplanatic” (1857), f&#8202;/15 to f&#8202;/30

. 15.—Grubb’s “Aplanatic” Lens.. 16.—Rapid Landscape Lens. Long Focus.

(fig. 15); J. H. Dallmeyer’s “Wide Angle Landscape Lens” (1865), f&#8202;/15, angle 75°. In it distortion was reduced and marginal definition

. 17.—Rectilinear Landscape Lens. improved. “The Rapid (long focus) Landscape Lens” (1884), f&#8202;/12, angle 40° (fig. 16), was a modification of it, and at f&#8202;/8 is useful for heads in portraiture. W. Wray’s “Landscape Lens” (1886), f&#8202;/11, is also useful for portraiture in the larger sizes at f&#8202;/8. Fr. Voigtlander’s “Wide-Angle Landscape Lens” (1888) 