FOCUS) AND ORTIGS 43 
100 X 6? = 3600 in. = 300 ft. The hyperfocal distance, 
which is always equal to the focal length multiplied by the 
diameter of the aperture, and divided by that of the circle of 
confusion, is therefore for a 6-inch lens at f:6, 300 +6 or 
50 ft. When focussing on 10 ft., the near depth is (50 X 10) 
~ (50-+ 10) —81/3 ft. The limit of far depth is (50 X 
10) + (50 — 10) = 12% ft. When focussing on infinity, the 
nearest object in focus is at the hyperfocal distance, and depth 
extends from that point to infinity. When the hyperfocal 
distance is focussed on, the nearest object in focus is at half 
the hyperfocal distance, and the farthest at infinity, so that 
depth extends from half the hyperfocal distance to infinity. 
FocussING RULES FoR Hanp Cameras.—The following 
two simple rules may be useful to hand camera workers. 
1. Applicable when the background is not very distant: Focus 
on a distance equal to twice the product of the greatest and 
shortest distances, divided by their sum. Example: Suppose 
the subject to be a street scene with a house 20 yards away, a 
man 5 yards away, and both are required to be sharp; then 
(20x 5) + (5+ 20) X2=8 yards. 2. Applicable when 
the background is infinity or very distant: Focus on a point 
just double the distance of the nearest point. Example: 
again suppose that the nearest point is 5 yards distant; then 
the point to focus on is 5 X 2 = 10 yards. 
The following rule may also be used: square the focal 
length of the lens in inches, multiply by 100, divide by the f 
number of the diaphragm, and then divide by 2. Example: 
Witthieaviens of 5. in. focus at #28; 5 xia 100 — 2500. 
2500 — 8 = 312%, and 312% ~2—156 in. Then every- 
thing beyond 156 in. (13 ft.) will be in focus if this plane 
be focussed on. 
CoMBINING LEeNsEs.—To find the focal length of two 
lenses separated by a short distance, multiply the focal lengths 
