Unfortunately, manufacture of the rectilinear by the introduction of chromatic aberration, so that the lenses are sometimes so used for portraiture. No correction for focus is then necessary.
An important advantage of the rectilinear lens for the possessor of a camera having suffi cient bellows extension is the possibility of using the back combination alone, forming a single achromatic lens of double the focal length of the complete lens. The largest aperture, F/8, is then actually F/16 or a little less. The image is almost twice as large as that obtained with the complete lens, but is only one-quarter as bright. A little distortion may be seen at the edges of the field, which need not prevent its use as a landscape lens. The rectilinear is thus the cheapest and simplest convertible lens.
100. Wide-angle Rectilinears. The somewhat large separation between the components of the rectilinear lens restricts the angle of field illuminated, and it is impossible, even by stop ping down, to increase the angle to the extent required in many classes of work. In the 'sixties were introduced the Globe lens of Harrison and Schnitzer (1863) and the Panto scope of Busch (1866), symmetrical combinations of two achromatic lenses of the form shown in Fig. 65, covering 9o° and at maximum apertures of F/17 and F/3o, with considerably less curvature of the field in the latter case. Steinheil subsequently succeeded, by modifying his aplanat but keeping the same principle, in making his wide-angle aplanat (Fig. 71), cover ing 105° at F/18, showing marked superiority over its predecessors, particularly from the point of view of spherical aberration.
Although in principle the wide-angle recti linears made on this classic construction are convertible, their single components, which have an aperture of only F/36, are too slow for this convertibility to be of any practical use.
matter of corrections, as each of the two com binations must be separately corrected for several aberrations. With the glasses available at this period progress was only possible by giving up both symmetry and the separate correction of the individual components, each component, on the contrary, being left with considerable aberration, which compensated that in the other. This fruitful conception was first applied with real success by A. Steinheil (1881) in his group antiplanat (Fig. 73), covering 62° at the central region being remarkably' well corrected for astigmatism. The consider able weight of this lens was a serious drawback to its use on the light cameras which began to 101. Anachromatic Symmetrical Lens. This lens, the use of which was recommended by Puyo and de Pulligny (1903), is based on the periscope of Steinheil, but, there being no need for a large angle of field, the aperture was increased from F/4o to F/6-5. The symmetrical anachromatic
consists of two identical convergent menisci placed one on either side of the stop, with their convex surfaces outwards and separated by at least one-sixth of their common focal length (Fig. 72). The diameter of the glasses being almost one-tenth their focal length, and con siderable latitude being allowable in their separ ation, these can be substituted for the com ponents of a Petzval type of lens, so that mountings made in bulk can be used. When the necessary correction is made for focus,' very agreeable portraits can be made, covering a field of about 30° 102. Antiplanats. A symmetrical construc tion reduces the resources of the optician in the be fashionable with the introduction of the gelatino-bromide plate. Hence this lens and several variants of it made by R. Steinheil merely aroused curiosity, the more so as the approaching appearance of the anastigmat was to furnish a complete and more elegant solution of the problem of the photographic objective.
103. The First Anastigmats. The principles of the correction of astigmatism had been laid down in 1843 by Petzval, but none of the glasses available to opticians at that time allowed these conditions to be satisfied. The appearance of new glasses, heavy crowns, and light flints, enabled P. Rudolph, at the instigation of E. Abbe, to design, after less successful attempts, a type of unsymmetrical, unconvertible doublet, the appearance of which (1890) marks an event in the history of photographic optics, at least as important as the invention of the portrait lens or the aplanat. Each of the components of the different series of lenses made on the same principle (afterwards known as Protars) was formed of two cemented glasses, the dimensions, curvatures, and thicknesses varying as the requirements were for a lens of great rapidity (field of 8o° at F/7-2) or one of very large field (Ile at F/i8). Fig. 74 shows this type of wide angle lens, which is still made to this day. A much better correction of astigmatism was obtained in 1893 by adding a glass to the rear component (Fig. 75), this lens covering perfectly, without curvature of field, an angle of at F/8 and still satisfactorily 75° when stopped down to F/22. In 1901 H. L. Aldis showed that an excellent image could be obtained with a considerably simpler construction (Fig. 76), covering a field of 51° at F/6 and 90° at F/32.