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HANDBOOK OF PHOTOGRAPHY 



the system from a given object point. It may be a variable-iris diaphragm, as in a 

 photographic lens, or it may be a lens cell as in a telescope or a projection lens or a 

 microscope objective. By analogy with the eye, this stop is called the "iris" of the 

 system. Bj- further analogy with the eye, the image of the iris as seen from the 

 entrance end of the system is called the "entrance pupil," this pupil being therefore 



the common base of the cones of rays entering the 

 system from various points in the object plane. 

 Similarly, the image of the iris formed by that part 

 of the lens system lying between the iris and the 

 image is called the "exit pupil," which is thus the 

 common base of all the cones of rays proceeding 

 from the lens to various points in the image. The 

 entrance and exit pupil are evidently images of each 

 other, since they are both images of the iris. 



The positions of the iris and the pupils in a symmetrical lens are indicated in 

 Fig. 14, together with a typical beam of light passing through the system from a 

 distant object to the image. 



Fig. 13. — Diagram illustrating 

 the nodal slide. 



Fig. 14. — The iris and pupils of a symmetrical lens. 



The Brightness of Optical Images. — In Fig. 15 is shown a lens represented merely 

 by its two pupils. If the extreme entering ray from an axial object point has a slope 6 



Fig. 15. — Passage of light through a lens. 



and the emerging ray a slope 6', then the iUumination E (foot-candles) on the photo- 

 graphic plate at B' is given by 



E = kwB sin 2 d' (9) 



Here B is the intrinsic brightness of the object (in candles per square foot), and k is 

 the transmission of the lens. "Transmission " is defined as the ratio of the amount of 

 light leaving the lens to the amount entering it and is always less than unity; it will be 

 discussed more fully on page 23. 



