226 HANDBOOK OF PHOTOGRAPHY 



lens consists of a positive doublet lens, such as are found in all good cameras, behind 

 which is placed a compound negative lens at approximately half the distance of the 

 focal length of the positive element. The telephoto lens may be constructed as a 

 complete lens system, in which case fixed magnification is obtained, or it may be made 

 by adding the negative element to the existing camera lens. In the latter case the 

 size of the image is variable, since image size depends upon the separation between 

 the two elements, which separation in turn is variable. 



Fixed telephoto lenses are focused in the usual way, and no difficulty is encountered 

 in determining the exposure. The procedure is the same as determining the exposure 

 for any ordinarj^ lens, since the equivalent focal length and aperture stops are specified 

 by the manufacturer. 



For telephoto lenses having variable focus, the determinations of exposure and 

 magnification are not so simple as in the case of fixed-focus lenses. If Lp is the focal 

 length of the positive lens element, L,, is the focal length of the negative lens element, 

 M is the linear magnification obtained, and d is the bellows extension, the equivalent 

 focal length of the combination is 



Lf = MLj, (38) 



and the magnification obtained is 



M = 1 + ^ (39) 



■Lin 



The equivalent aperture stop of the telephoto lens /e in terms of the aperture stop 

 marked for the positive lens element fp is 



/. = Mfp = fp{l + £) 



(40) 



It should be noted that for any given magnification the equivalent aperture of the 

 telephoto lens is alwaj^s M times the aperture stop of the positive element. 



For example, if we have a 15-cm. positive lens with an aperture stop of //5.6, a 

 negative lens of 8 cm., and the bellows extension of 24 cms., the magnification is 



M = 1+24^ =1+3=4 (41) 



The focal length of the combination of lenses is 



L/ = 4 X 15 = 60 cm. (42) 



and the maximum aperture 



/e = 4 X 5.6 = 22.4 (43) 



When the equivalent aperture stop is determined, the telephoto lens may be used 

 the same as any other lens and the exposure is determined with tables and exposure 

 meters in the usual manner. 



Absorption and Reflection in Lenses. — The amount of light transmitted through a 

 lens depends not only upon the /-number and quantity of light incident upon it but 

 also upon the amount absorbed in passing through the individual elements and that 

 reflected from its various surfaces. The light transmission decreases as the number of 

 elements of a lens increases, so that for a given aperture stop a simple lens may easily 

 transmit more light than a more complicated and more highly corrected lens. Thus, 

 so far as light transmission at a given aperture is concerned, a meniscus lens may 

 transmit more light than a Tessar or Plasmat, and consequently may be a "faster" 

 lens. Since the amount of light transmitted through the lens determines the exposure, 

 it is apparent that all lenses do not produce the same exposure under identical light 

 conditions and when used at the same aperture. It is, of course, true that the loss of 



