THE DEVELOPMENT OF THE PHOTOGRAPHIC OBJECTIVE 



43 



Stigmar//6.3, the Bausch and Lomb Aero lenses, and the Zeiss distortionless Orthom- 

 etar//4.5. Some advantage has been gained by separating the old and new achromats 

 in the double-protar type, for instance in the new Taylor-Hobson Series 15 con- 

 vertible anastigmat //6.8 (Fig. 23). The Schneider Ortho-Angulon contains two 

 symmetrical quadruplets in which the inner elements have bieen detached (Fig. 24). 

 The Meyer Kino-Plasmat //1. 5 is an extreme variant of these nearly sj^mmetrical 

 types (Fig. 25). 



The Cooke Lens. — These recent developments represent the limit to which the 

 wide-angle symmetrical "continental" type of lens has been carried. At the other 

 end of the scale, we have the Petzval portrait lens and its variants, which have a large 

 aperture (up to about //2. 4) but cover only a rather small field (20°). The limit along 

 this line is the recent Zeiss R-Biotar for motion-picture photography of X-ray images 

 on fluorescent screens, which has an aperture of //0.85 and covers a field of only 

 14° (Fig. 26). 



In order to provide a lens having intermediate properties between these "conti- 

 nental" and "Petzval" tj^pes, H. Dennis Taylor in 1893 and succeeding years devel- 



FiG. 26. — Zeiss R-Biotar for mo- 

 tion-picture X-ray photography. 



Fig. 27.— Cooke 

 lens with negative 

 element mounted be- 

 tween positive ele- 

 ments. 



Fig. 28. — Beck 

 Neostigmar, of the 

 Cooke type. 



oped the well-known Cooke lens in which the Petzval sum is reduced by separating 

 the constituents of an achromat.^ A simple separated doublet would suffer from 

 very bad distortion and transverse color; therefore, Taylor divided the positive 

 element into two and mounted the negative element between them (Fig. 27). 

 Although it is theoreticallj^ possible to design such a lens using old glasses, he shortened 

 the system and made the corrections easier by using barium crown instead of ordinary 

 crown for the convex elements. This type of lens can be made in apertures up to 

 //3.5, to cover a field of 55°, and has the great advantages of weak curves, few com- 

 ponents, and no cemented surfaces. At the same time it has six glass-air surfaces, 

 but if the iris is placed in the rear airspace no trouble from ghosts or flare spots is 

 ordinarily encountered. The Cooke t3'pe of lens is made by many firms and has been 

 regarded as the major real invention in lens design since the advent of the new glass 

 types. The Beck Neostigmar is really of the Cooke type, with the diaphragm placed 

 in the front airspace (Fig. 28). 



The aperture of the Cooke lens was raised to /,'2.3 in 1925 by Bielicke in the 

 Astro Tachar lens (U. S. Pat. 1540752), and in 1926 by Lee in the Taylor-Hobson 

 //'2.5 Speedic lens, hy splitting the rear positive element into two closely spaced posi- 

 tive lenses (Fig. 29). Even the recent Zeiss Sonnar lens (Fig. 30) covering 54° at 

 //1. 5 may be regarded as a development of the Cooke type in which both the middle 

 negative lens and the rear positiA^e lens have been made into cemented triplets, 

 although, of course, it by no means follows that this design was actually arrived at 

 by successive modifications from the original Cooke tj'pe. 



1 Taylor, H. D., Optical Designing as an Art, Trans. Optical Soc. (London), 24, 143 (1923). 



