432 



HANDBOOK OF PHOTOGRAPHY 



The focal lengths of the objective and the condensers should be of the same general 

 dimension or, perhaps, the objective may be somewhat greater in focal length than the 

 condenser. 



An approxiniate expression for the distance between lens and easel is {M + 1)/, 

 where M is the magnification and 'f is the focal length of the objective. 



Degree of Enlargement. — With any enlarger the degree of enlargement increases 

 as the distance between lens and easel increases. The formulas below relate the 

 magnification and the distances between negative and lens and lens to easel. If / is the 

 focal length of the projection lens, do is the distance from negative to lens, and Di is 

 the distance from lens to paper, the sum of the reciprocals of these distances must 

 equal the reciprocal of the focal length of the lens. Then the magnification is the ratio 

 between the two distances, 



Ui+i (6) 



/ do ' Di 



M = 



Di 



(6) 



Ref/ec:^or~^ 



Lamp- 



For practical purposes, however, it is simpler to use a table like that shown on page 

 433, worked out from these relations. 



Objective Lens. — The lens should be an anastigmat of flat field and fairly large 

 aperture. It should be equipped with a diaphragm, but if considerable light is 

 employed, this diaphragm should not be made of material that will easily warp from 

 the resultant heat generated. Since the condenser concentrates the beam of light 

 coming through the negative, the amount of light through the enlarger lens does 

 not vary with the /-number in the same manner as in a camera. A lens stopped down 

 to //8 may pass as much light as one marked //4.5 because of this concentration. 

 Therefore one must make trial exposures each time the aperture or magnification is 

 changed. If, however, exposures are not too short to be controlled at full aperature, 

 there is no object in stopping down the lens. 



The lens from any camera that will bring rays to a sharp focus may be used in an 

 enlarger. A better lens, of course, will produce better definition, but many a good 



camera lens will make a good enlarger lens. 

 Lenses especially made for enlarging work are 

 often better corrected over a small angle than 

 camera lenses. The lens must be capable of 

 "covering" the desired negative. 



Modern miniature enlargers are often designed 

 so that the photographer can use a lens inter- 

 changeably for camera or for enlarger, thus sav- 

 ing himself the expense of owning two lenses. 



Diffusion Enlargers. — In this equipment the 

 light from the source is scattered through one or 

 more diffusing mediums, which may be ground 

 glass or flashed opal glass. Because of the scat- 

 tering of the light, rays arrive at the negative 

 from all angles with respect to the surface of the 

 negative. The illuminated diffusing medium 

 becomes the light source for the negative. 

 Scratches and flaws are minimized. 

 According to Tuttle {Journal of the Optical Society of America, October, 1934), the 

 difference in contrast between complete diffusion obtained by heavy pot-opal glass 

 and no diffusion amounted to about 3 to 1 when Eastman Supersensitive Panchromatic 

 film was used as the sensitive material. 



Gro und glass 7^ 



Oq 



Enlargemeni- 

 oneo/se/--^ 



Fig. 



11. — Diffusion enlarger of the 

 vertical type. 



