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HANDBOOK OF PHOTOGRAPHY 



photographic effect, when photographing an object with a camera and lens system. 

 However, the D-logio E characteristics are usually expressed in terms of density and 

 meter-candle-seconds. The illumination of the object being photographed is not ordi- 

 narily determined in meter-canc^e-seconds, and even if this were possible, through the 

 use of properly calibrated exposure or illumination meters, the intensity of illumination 

 on the plate is vastly different from that of the original object because of the reduction 

 in size, the effect of the aperture stop, the focal length, and other characteristics of the 

 lens system. To make maximum use of the sensitometric concepts and to under- 

 stand fully the various and numerous factors which enter into exposure, it is desirable 

 to provide the connecting link which relates the exposure, as given in the sensitometric 

 sense of the term, and the brightness of the object as this may be determined by 

 measurements with an exposure or illumination meter. It is proposed to construct 

 this connecting link based upon theoretical considerations for two reasons: (1) An 



L(l+q) 



|X|=|L[Cl+c^)-f^]|— - 



-> 



<-- 



Fig. 3. — Optical system of a camera showing axial rays. The luminous intensity, /', 

 of the point P' on the photographic plate can be expressed in terms of the luminous intensity, 

 I, of the point on the subject, P, and the characteristics of the lens system. 



understanding of the theory of exposure provides an excellent basis for understanding 

 the practical treatment which is to follow; (2) the relations and equations which are 

 derived from theoretical considerations are required for a full explanation of the use 

 of exposure tables given in another section of this chapter. 



Image Brightness as a Function of Optical System. — It is now necessarj'^ to estab- 

 lish the connecting link by which the brightness of the image on the photographic 

 plate may be determined from the illumination of the original object being photo- 

 graphed. This link involves the optical system of the camera, which, so far as 

 exposure is concerned, includes the iris diaphragm, the bellows extension, a filter 

 (if one is used), and the shutter, as well as the lens system proper. 



Let Fig. 3 represent the lens system of the camera, in which a point object P, 

 whose luminous intensity is /, produces an image object of itself P' with luminous 

 intensity I' on the photographic plate. The iris diaphragm or aperture is repre- 

 sented as being at A. The principal planes of the lens are represented as lying at PP 

 and at PP', while the entrance and exit pupils are designated as being at NP and XP, 

 respectively, and the principal focal lengths are L and L'. 



The point P may be self-luminous or may be illuminated by reflected light. In 

 either case it will illuminate the entrance pupil of the lens NP with an intensity 

 inversely proportional to the square of the distance between P and R, the latter being 



