50 



HANDBOOK OF PHOTOGRAPHY 



I 



Fig. 



67. — Dallmeyer-Bergheim 

 diffuse portrait lens. 



Portrait Lenses. — Any lens of sufficient aperture may be used for portraiture. 

 Moreover, as was mentioned on page 26, good perspective demands a long focus, 

 consequently portrait lenses are usually somewhat large in size. The field to be 

 covered in portraiture is usually very small, and conseqiiently a lens of the Petzval 

 type is really surprisingly satisfactory. 



For artistic photography, there is a considerable demand for "soft-focus" or 

 "diffusion " effects. These effects can be obtained by placing over the lens a diffusion 



attachment, which is merely a glass disk carrying 

 ridges or other means for diffracting a portion of 

 the light. Some lenses are, however, specially 

 designed to give a softness of contrast by deliber- 

 ately leaving a suitable residual of spherical or 

 chromatic aberration, or both, in the design. It 

 is essential for this purpose that the definition 

 itself shall be good, but there must be a softness 

 or diffuseness of the light superposed on the image. In some lenses variable diffusion 

 may be obtained by sliding one lens along the barrel. In other cases, the diffusion 

 can be varied by stopping down the iris diaphragm. An interesting design for 

 diffuse portraiture is the Dallmeyer-Bergheim lens, which consists of a telephoto 

 combination of two simple lenses with variable separation to give variable sizes of 

 pictures (Fig. 67). 



Anamorphic Systems. — It is occasionally desired to have a lens which will give 

 different magnifications in two direc- 

 tions. ^ This is possible by means of 

 cylindrical lenses used in the manner 

 indicated in Fig. 68. If the two lenses 

 are equal in power and arranged with 

 axes perpendicular to one another, the 

 magnifications in the directions of the 

 cylinder axes will be in and 1/m, respec- 

 tively. The value of m will become greater, the greater the separation between the 

 lenses. The relative values of the magnifications can be altered by using two lenses 

 of unequal focal length. If one cylindrical lens is rotated relative to the other, the 

 image of a square becomes distorted into a rhomboid. 



Supplementary Lenses. — -These are lenses intended to be attached to the front 

 of an existing lens to lengthen or shorten the focal length. They are generally simple 



Fig. 68. — An anamorphic optical system. 



Fig. 69. — Passage of light rays through a portrait-attachment supplementary lens. 



meniscus landscape lenses, designed to work with a stop situated at the entrance-pupil 

 of the existing lens, as indicated in Fig. 69. If the supplementary lens is positive, 

 the over-all focal length is shortened, and on a focusing camera the picture seen will be 

 reduced in size. With a fixed-focus camera such as a cheap box camera, the addition 

 of a positive lens in this way enables close objects to be photographed. The correct 



1 For a full discussion of the various means by which this may be attained, see Newcomer, U. S. 

 Pat. 1932082 a933) ; 1945950 and 1946951 (1934). 



