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1949 OBJECTIVE LENSES 91
collective meniscus immediately following the first doublet, a procedure favorable to the reduction of spherical aberration. Fig. 2c shows the Taylor, Taylor, and Hobson //0.8 radiographic lens,10 a design in which the rear component of the basic Petzval configuration is split into two doublets, again taking advantage of reduced spherical aberration through dividing. The//0.6 lens of Kaprelian,11 Fig. 2d, adds another element to the R-Biotar upon which it is based in an attempt to obtain better spherical and chromatic correction and to increase the back focal length by a shift in power.
Another group of large-aperture objectives is based upon the excellent six-piece Gauss type, the principal defects of which are higher order astigmatism and oblique spherical aberration. Leitz,12 Fig. 2e, has utilized a modification of the Gauss type which comprises the addition of two crown elements, one before the dispersing menisci and one after the menisci, to*extend the aperture to //0.85. The 7. 5-centimeter Leitz objective has an acceptable field of about one inch diameter and covers the 35-mm frame for cine fluorography. An //I lens produced by Wray for radiography is of the same general type ; here the additional collective element has been provided only at the long conjugate end and the second dispersive element appears as a singlet. The Wray lens, in common with most objectives intended for radiography, is especially corrected for use at short conjugates,
An //0.95 Gauss-type objective of Zeiss13 provides additional power through the provision of a second rear crown and includes a field flattener close to the focal plane, Fig. 2f. Herzberger14 has designed an objective, Fig. 2g, of //0.8 aperture which utilizes a front group comprising a complete, well-corrected Gauss objective followed by a rear-collective group of three elements based upon a system originated by Luboshez.1516 The Herzberger objective provides for generally good correction. The Astro Tachon //I of Bielicke17 utilizes the front components of the Gauss type in combination with rear components derived from the air-space type. Bielicke's //I design has been modified by the addition of another element to produce the;/0.95 Tachon, Fig. 2h.
Another group of extreme-aperture objectives derives from the Gauss type to form a separate and distinct class which results when the second dispersive meniscus is removed, the last element made strongly collective, and a meniscus introduced between the front element and the dispersive meniscus. Lee,18 Fig. 2i, achieves an aperture of //I by this approach and in another form, Fig. 2j, also of //I