Ch. VII] 



FOCUSIXC IN' PHOTOGRAPHY 



21 I 



focused on the central scratch, which must be in the exact plane where 

 the sensitized photographic surface will be during the exposure. If 

 now an object is brought to an accurate focus at this plane, it will 

 also be in focus on the sensitized surface of the dry plate. Except 

 for aid in arranging the object and for general focusing, the frosted 

 glass can be entirely omitted, and a 

 focusing glass giving about 8 to 10 

 diameters magnification is set in a 

 board which takes the place of the 

 ordinary frosted glass screen. This 

 is put at the level to bring the focus 

 exactly at the plane where the sensi- 

 tive surface of the negative is to be. 



The position of the focusing glass 

 is determined as follows: 



The plate holder with a clear glass 

 plate or a thin negative is in the 

 holder. And on the film side is a 

 diamond scratch or an India ink 

 mark near the middle of the face 

 usually occupied by the sensitive film. 

 It is very important that the mark 

 should be on the side occupied by 

 the film. 



The scratch or ink mark is a guide 

 for getting the focus at the right 

 level. Now with a tripod or other 

 magnifier, preferably with the mag- 

 nifier to be used later, get the image focused of the metric scale and 

 its explanation or other sharp print exactly on the surface where 

 the diamond or ink mark is. To make sure that there is no better 

 focus obtainable it is worth while to make a negative of the printed 

 matter used for focusing. On the excellence of the focus determined 

 depends the excellence of all future pictures which will be made. This 

 method has the further advantage that the focus level is determined 

 for the plate holder and not for ;i focusing screen. It is in fact an 



Fig. 127. Ground-glass Fo- 

 cusing Screen with Clear Cen- 

 ter for Fine Focusing. 



1 The ground or frosted sur- 

 face of the glass. 



2 A cover-glass stuck to the 

 frosted surface with Canada Bal- 

 sam. This renders the frosted 

 surface transparent. 



x Pencil mark in the center of 

 the focusing screen on the frosted 

 surface t<> serve as guide when 

 focusing with a magnifier. 



