42 PHOTOGRAPHIC FACTS AND FORMULAS 
will appear sharp in front of and behind the object focussed 
for. The following table shows the distances for a 6-inch 
lens starting with f:8, though in the scales only the nearest 
whole numbers are given: 
100 K 62. f:8 = 450 in. — 37 ft. 6 in. 
1000007’ f: 165 225 ta 00 Tea 
100 « 6° =~ f:24 = 150 in. — 12 ft. 6 in. 
100 & 6? + f:32 = 112Y in. — 9 ft. 4 in. 
100 & 6? + f:40 = 90 in. — 7 ft. 6 in. 
FocussInc SCALE.—To construct a focussing scale, draw a 
line equal in length to the equivalent focus of the lens, mark 
off exactly the half, third, fourth, sixth, eighth, etc., and add 
1 to each division; this will give a scale as below. Multiply 
the numbers below the line by the focus of the lens, and the 
result will be the distances in inches at which an object will 
be sharp, if the lens be placed at this point. To fix this scale 
to a camera, focus first on the clouds, or a very distant object, 
mark the baseboard at any convenient part of the camera 
front, and place B even with this mark, the scale extending, 
obviously, from this mark away from the rear of the camera. 
3 4 5 79155 
Fig. 5 
DeptH oF Focus.—Welborne Piper suggests that, know- 
ing the hyperfocal distance for the stop in use and the 
distance of the object in sharp focus, if we divide the product 
of these two distances by their sum, the result will be the 
distance of the nearest object in focus; if their product is 
divided by their difference, the result is the distance of the 
farthest object in focus. Example: suppose we are focussing 
with a 6-inch lens on an object 10 ft. away, taking 1/100 in. 
as the circle of confusion. The depth constant for the lens is 
