Page:EB1911 - Volume 21.djvu/537

consists of two unsymmetrical combinations, each formed of two single lenses of very transparent glass, dense baryta crown and light flint, separated by positive and negative air-spaces (fig. 44). The separate halves cannot be used as single lenses, neither being fully corrected for colour. It is well adapted for portraiture, groups or landscapes, especially for rapid hand camera work, on account of its covering power, with freedom from astigmatism and sharp definition with large relative aperture.

Fig. 43.—Planar. Series Ia. f /4.Fig. 44.—Zeiss’s “Unar.”

In 1898 Messrs Goerz patented their “Double Anastigmat Celor,” series Ib, f /4·5 to f /5·5. It is a symmetrical doublet, each element consisting of two thin single lenses: a positive of high and a negative of low refractive index, separated by an air-space (fig. 45). It is derived from the triple anastigmats by decreasing the refractive power of the central convex meniscus to the refractive (power of air, so that it becomes a convex air-space between a ouble convex and a double concave lens. Less deeply curved surfaces can be given to the lenses, and the doublet gives an astigmatic flatness of field over an angle of 62° to 66°, equal to the best anastigmats with a still larger aperture. Series Ic., f /6·3, is similar and recommended for hand cameras, the aperture being smaller.

Fig 45.—Goerz’s “Celor”Fig 46.—Goerz’s “Alethar”

Goerz’s “Hypergon,” (1900) f /22, angle 135°, is a symmetrical doublet of remarkable construction, consisting of only two single semi-globular, very thin lenses, with diaphragm at the centre of curvature between them. Astigmatism and curvature have been eliminated, and definition is good over the above wide angle with no distortion. Chromatic aberration is uncorrected, but compensated for by using a small sto . A star mask is fitted in front of the lens to allow for falling off of illumination towards the margin (E. Jb., 1901, p. 103). The “Syntor” (1903), Series Id., f /6·8, angle 64° to 70°, is on the same principle as the “Celor,” but cheaper, for use in hand cameras or telephoto combinations. The “Alethar,” series V. (1903), f /11, is a lens with diminished secondary spectrum, for three-colour reproductions, half-tone process work, and general purposes. It is a symmetrical doublet, each element consisting of a negative and positive separated by an air-space (fig. 46). The negative is composed of three cemented lenses, which correct the spherical and chromatic aberrations more fully than hitherto possible, so that all the colours of the spectrum are focused in the same invariable plane. It gives great crispness of definition at full aperture (W. Zschokke, E. Jb., 1904, p. 165). Goerz’s “Pantar,” f /6·3 (1904), is a convertible 4-lens anastigmat, and an improvement on the “Dagor,” in that the single elements are completely corrected for coma, and thus form efficient long-focus lenses for landscape, &c., at an aperture of f /12·5, while the doublets formed by various combinations of the single elements are universal ob]ectives working from f /6·3 to f /7·7. The single elements are similar to those of the “Dagor,” but have an additional negative lens at the back, so that the outer two of the three cemented surfaces have a collective and the inner one a dispersive action, by which coma is eliminated (E. Jb., 1905, p. 55).

Fig. 47.—Aldis Lens. Series II. Fig. 48.—Aldis Lens. Series III.

In 1902 H. L. Aldis issued the “Aldis Lens,” f /6, a doublet composed of a cemented meniscus in front and a single double convex back lens. It is a long-focus objective with short back focus, and is made in two forms, series II., f /6 (fig. 47). and series III. (1903), f /7·7 (fig. 48). In the latter the back element is very thin, and the front combination of infinite focal length. By discarding the symmetrical form simplicity is secured, while open or reflecting surfaces are avoided. Special attention has been paid to perfect correction of spherical aberration in the centre of the field. It is lighter, smaller and cheaper than series II. The “Duo” lens of the same maker (1907) is intended to replace the front lens and double the focus, but with less rapidity and without any loss of quality. The “Trio” (1908) is similar, but only increases the focus one and a half times and is thus more suitable for cameras of short extension. The Aldis “Oxys” anastigmat, series II. (1903), f /5·65, angle 35°, is an improved form. Being an unsymmetrical cemented doublet it is free from the defects incidental to air-spaces and is constructed to give more perfect correction for flatness of field with large aperture and wide angle.

It is generally stated that it is impossible to make a spherically, chromatically and an astigmatic ally corrected photographic objective with the old optical glasses. K. Martin, of Messrs Busch of Rathenow, has, however, shown (E. Jb., 1902, p. 68) that it is quite possible to do so with a system of separated lenses, and that it is immaterial whether the index of the flint or the crown is the higher. An anastigmat on this principle was issued by Messrs Busch in 1902, as the “Omnar,” series III., f /7·7 (fig. 49). Series II., f /5·5, angle 75°, and I., f /4·5, have since been issued.

Fig. 49.—“Omnar,” Series III.Fig. 50.—Ross’s “Homocentric.”

It is a symmetrical doublet, each element consisting of a negative flint meniscus of higher refraction, and a positive crown of lower refraction with an air-space between them in the form of a negative lens. The back element can be used alone. The “Lumar” series, by G. Rodenstock, is similar. In 1902 Messrs Ross brought out the “Homocentric,” a symmetrical doublet, each element consisting of a negative and positive meniscus separated by an air-space (fig. 50). It is constructed so that all rays of light emanating from any one point of the object are converged again into one point in the image. It is also quite free from spherical zones, is not altered in focus with different diaphragms, and thus has exquisite defining power. The colour correction is so perfect that the different coloured images are identical in size and position, thus rendering it specially suitable for three-colour and process work. The back lens can be used alone, with diaphragms, as a single lens of about double the focus of the doublet. It is made in several series: II., f /5·6, and III., f /6·3, for rapid and instantaneous work; V., f /8, for ordinary purposes; VI., f /8, for process work and three-colour reproduction. A later series, IV. (1907), “Compound Homocentric,” f /6·8, differs from the others in being a symmetrical doublet composed of two triple cemented elements, very close together and separated by a diaphragm.

Fig. 51.—Zeiss’s “Tessar.”Fig. 52.—Voigtländer’s “Heliar.”

It is specially suitable for outdoor work, also for copying and enlarging, having good covering power. Zeiss’s “Tessar” (1902) is a rapid unsymmetrical doublet, formed of two separated uncemented positive and negative lenses in the front element and a cemented meniscus at the back (fig. 51). The two halves cannot be used separately. The glasses used are very transparent, permanent and lessen the secondary spectrum. Three series are made by Messrs Ross, Ic., f /3·5 for cinematographic work and portraiture, and f /4·5 for hand camera work and portraiture; IIb., f /6·3 for general purposes, and VIII., the “Apochromatic Tessar,” specially corrected for three colour work and reproduction. They all give fine definition over a large flat field, free from any zonal aberration. The f /3·5 portrait lenses, with double the field and covering power of the Petzval lens, are an astigmatic and free from distortion. Messrs Voigtlander’s “Heliar” (1902), f /4·5, angle 50°, calculated by Dr H. Harting, is an objective of large aperture, suitable for portraits and very rapid instantaneous work, being well corrected for astigmatism, coma and curvature of field, with freedom from flare. It is a triplet consisting