38 PHOTOGRAPHIC FACTS AND FORMULAS 
is available, it is only necessary to measure the size of the two 
images, when both lenses are focussed on the same distant 
object, and the focal lengths are proportional to the sizes of 
the two images. 
A Geometrical Method.—Focus two distant objects, as 4 
and B in Fig. 1, and let C and D be the images of these 
A 
ee 
objects; then f = h-~tan a, a being the angle between AC 
and BD. Measure the length h, the distance between the 
images of the two objects, and also CL, their distance from 
the lens; then f =h~(CD—~CL). Let CD or h=4 in. 
and CL = 8 in.; then f = 4~+(4-8) =8. 
Another Geometrical Method—At a distance a, at least 
100 times the focus, set off at right angles to the axis of the 
lens two marks %4 a distant from the axis. The distance 
between the two images on the screen will be 14 f. 
Grubb’s Method—At each side of the focussing screen 
make a pencil mark equidistant from the center. Place the 
camera flat on a sheet of white paper in front of a window. 
Focus on a very distant object, so that its image falls exactly 
on one pencil mark. Then draw a pencil line along the side 
of the camera, using the edge of the camera as a straight 
edge. Shift the camera so as to bring the image of the same 
object on to the other pencil mark on the ground glass, and 
again draw a line along the side of the camera, using the same 
