OBJECTIVES 59 



Photographic Lenses 



1" Goerz Anastigmat by Ross 

 5J" Homocentric Ross 



Dallmeyer's " Adon " Telephoto Lens 



Properties of Objectives. The following factors must 

 all be taken into account when selecting objectives, 

 and many of the points can be made out by a careful 

 examination of one or other test object ; but it must 

 be borne in mind that it is usually necessary to make 

 a compromise when constructing a lens with a view 

 to general work. Some good property, almost essential 

 for one purpose, must be sacrificed to a slight degree 

 to gain others more necessary for other purposes ; in- 

 stances of this are mentioned. 



Focal Length and, Power of Objectives. Objectives 

 are designated either by their " equivalent focal lengths " 

 or by certain arbitrary letters or numbers, and the 

 term equivalent focal length calls for some explanation. 

 It is possible to imagine the substitution of an " equiva- 

 lent " simple lens for any compound lens, so that the 

 simple lens will have the same magnifying effect as 

 the other ; but the path of the rays through the com- 

 pound lens can only be reproduced by the simple lens 

 if it is further imagined to exist in two different planes 

 at once. In the first plane it may be supposed to receive 

 the rays from the object, in the second to transmit them. 

 The focal length of this simple lens is taken as the 

 " equivalent focal length " of the objective, and the 

 two planes in which its focus is supposed to exist are 

 the upper and lower focal planes of the objective. The 

 power of an objective varies inversely as its equivalent 

 focal length. 



Magnification always refers to increase in linear 

 dimensions, and is more fully dealt with in connection 

 with the use of objectives and eyepieces together. An 



