POCUS AND VOLES 41 
measurement, but the distance between the marks on the 
ground glass must always be the same as the extension of 
the camera from the infinity mark. 
DEPTH OF Focus ScALE.—Decide what stops the scale is 
to be constructed for. Asa rule, it will be found sufficient 
to calculate for f:8, f: 16, f: 24, etc., as with larger apertures 
the scale becomes too confused, and one rarely uses a larger 
aperture than f:8 for snapshot work. Draw a scale each 
division of which is equal to one-hundredth of the aperture, 
this being the assumed diameter of the circle of confusion; 
if this degree of sharpness is not considered sufficient, one 
may adopt one-two-hundredth or smaller, but, obviously, the 
smaller the divisions the more confused the scale and the less 
value practically. Besides that, one cannot estimate the dis- 
tance of an object with sufficient accuracy for the smaller 
scales. Utilising the formula 100 < focus squared — ratio 
aperture (100 F* ~r), find the nearest points in focus. 
Mark off on the divided scale these distances, starting from 
the infinity point, as in Fig. 3. 
G 38 191 Ie 6 4 3% 
Fig. 3 
Construct another scale of like dimensions and divisions, and 
mark off with the stop apertures only, starting in the center 
as in Fig. 4. 
F64 F32 F16 F8 F8 F16 F32 F64 
Fig. 4 
By placing Fig. 4 over Fig. 3 and sliding it along so that the 
arrow in the center marks the distance focussed on, one can 
at once read off under the stop numbers the distances which 
