196 Dr Searle, The deternunation of the effective 
§ 2. Stop in front of lens. When the stop S (Fig. 1) is placed 
in front of the lens system, as was often the case in the older 
photographic lenses, its effective diameter for landscape work is 
Fig. 1. 
simply equal to the actual diameter of the stop itself If the 
diameter of the stop is measured and is found to be a cm., and if 
the focal length of the lens is f cm., the aperture ratio of the stop 
is a/f and thus, if the “f/” number is //n, 
WENO BURP RAA He acnoocs cusacac: (1) 
and | EN fC MMPMOR A Sing Sbeo0cc0%00%" (2) 
§ 3. Stop between components of lens system. A stop does 
more than merely regulate the amount of light passing through 
the lens. The position of a stop of given aperture has an 
important influence upon the effects of the five “defects” of a 
lens, viz. upon the effects of Spherical aberration, Coma, Astig- 
matism, Curvature of image surfaces, Distortion; in the case of 
modern lenses consisting of two separated components, it is found 
that it is advantageous to place the stop between the components, 
as in Fig. 2. The effective diameter of the stop will now not be 
equal to its actual diameter. The effective diameter can be cal- 
culated from the actual diameter when the position of the stop 
relative to the two components and also the optical constants of 
the components are known, but it would be troublesome to obtain 
all these data. It is better to treat the lens system as a whole 
and to arrange the measurements so that there is no need to take 
the lens system to pieces. 
Let S (Fig. 2) be the stop and let 7, be the image of S formed 
by the front lens Z. Then S is the image of 7, by the same lens. 
