THE DEVELOPMENT OF THE PHOTOGRAPHIC OBJECTIVE 49 



It is clear that a high degree of freedom from distortion is most necessary in a proc- 

 ess lens, especially in one used to copy maps or any other diagrams on which measure- 

 ments are to be made. All aberrations other than distortion and transverse chromatic 

 aberration can be eliminated by stopping the lens down to a small aperture, and the 

 two latter aberrations will also vanish if the lens is symmetrical and used at unit 

 magnification. However, in lenses suitably designed, these two aberrations can 

 be made negligibly small even for magnifications other than unity. 



For three-color process work, it is clearly essential to use a lens in which the 

 transverse chromatic aberration is very highly , corrected, since the size of picture 

 produced must be identical in red, green, and blue light. Such a lens is called an 

 "apochromatic process lens," and ordinarily requires very careful choice of glass in 

 its design. 



Enlarger Lenses. — In general, the requirements to be satisfied by an enlarging 

 lens are not essentially different from those of a process lens. If an ordinary camera 

 lens is used for enlarging, trouble may arise because such a lens has been designed for 

 use with a distant object, and in an enlarger it operates at a magnification of one or 

 two only. The effect of this may be that a lens having a well-corrected flat field 

 when used on a camera has a curved field on an enlarger, and the definition may 

 become worse still on account of coma and other aberrations which disappear when a 

 distant object is used. Stopping the lens down will assist all these defects except 

 distortion and transverse chromatic aberration, jis was mentioned above under 

 Process Lenses. Incidentally, for three-color separation work, an apochromatic 

 objective is really necessary, for even the residual longitudinal chromatic aberration 

 of an ordinary lens may give a blurred red image with perfectly sharp blue and green 

 images. 



Aero Lenses. — The lenses used in aerial photography fall into two groups, viz., 

 those for aerial surveying and those for military purposes. In the first group the 

 aperture need not be high as aerial surveying is carried out only in perfect weather 

 with bright sunlight, and it is doubtful if such photographs would be taken at an 

 aperture greater than //8 or //ll. On the other hand, distortion and to a lesser 

 extent coma and transverse chromatic aberration must be corrected to a very high 

 degree since extremely precise measurements are to be made on the photographs taken 

 from the airplane. It goes without saying too that the definition over the whole 

 picture must be very sharp. The achromatism should tend toward the green or yellow 

 regions of the spectrum as a yellow haze-cutting filter is invariably used in surveying 

 work. 



For military purposes, however, a speed of at least //4.5 is necessary since pictures 

 must often be made in poor light, but distortion need not be quite so highly corrected 

 as is necessary for map making. Nevertheless good distortion correction is valuable 

 if it can be obtained, for then the same lens can be used for surveying if desired. 



In all airplane work, a long focus is desirable to give a large-scale photograph when 

 taken at a considerable altitude. 



Lenses for Infrared Photography. — As was mentioned above under the description 

 of chromatic aberration (page 30), when a lens is achromatized by the use of crown 

 and fhnt glasses, there is one particular wavelength for which the focus falls closest to 

 the lens. If a lens is intended to be used specifically in the infrared, i.e., for wave- 

 lengths between 0.75 and 1.2 ix, the minimum focus should be at perhaps 0.9 or 1.0 n, 

 instead of being at 0.55 y. as in visual achromatism, or at 0.48 y. for photographic achro- 

 matism. Thus, lenses for infrared use must be heavily overcorrected chromatically. 



Since a lens achromatized in this way would be virtually useless in blue light, an 

 infrared filter is sometimes incorporated into the lens to prevent its use for other 

 purposes. 



