PRINTING PROCESSES 



431 



the negative to produce a smaller image, the light source must be moved closer to the 

 negative to get even illumination. These adjustments must be made with considerable 

 accuracy to secure good focus and even illumination. Practically, however, the light 

 source is never an exact point source. Rays that illuminate the negative are both 

 transmitted through the lens and scattered by the negative image and do not go 

 through the lens. 



In miniature enlarging equipment no attempt is made to simulate a point source. 

 An opal-glass bulb is used which presents to the negative, through the condenser, a 

 very large source of light. No attempt is made to adjust the distance from light to 

 condenser. The only adjustments, therefore, are the lens-to-negative and lens-to- 

 easel distances. 



.Condensing lenses 



Lighi source 



si 



Fig. 10. — Element of a condensing enlarging system. 



Condensing Lens. — The plano-convex lenses are mounted with the flat sides out, the 

 inner surfaces touching or separated by a small distance. The equivalent focal length 

 of the pair may be calculated from the formula 



/l X/2 



/ = 



(2) 



/i + /2 - d 



where/ = the combined focal length; 



/i, fi = the focal lengths of individual lenses; 



d = the distance between lens centers. 

 In operating the enlarger, the distance Di from the objective to the easel controls 

 the size of the image. To bring the image into focus on the easel, the distance do 

 between objective and negative must be changed. These distances are related as 

 follows : 



Di=fM+f (3) 



'^ = {{i)+^ 



(4) 



where/ = focal length of objective; 

 M = magnification. 

 The condenser enlarger requires more space than the diffusing type because of the 

 considerable distance between the light source and the condensers. Furthermore, 

 unless objective and condenser are chosen carefully with respect to each other, the 

 space required for the enlarger may become very great indeed. If the focal length 

 of the condenser is less than that of the objective and if large values of magnification 

 are to be used, the distance between objective and easel may be such that the enlarger 

 cannot be accomodated within the darkroom. Thus, if a condenser with equivalent 

 focal length of 6 in. is used with an objective of 12 in., a magnification of 10 diameters 

 will require some 13 ft. for the distance between condenser and easel. 



