Vol. XXIV. 
1925 
Camera Craft. 
297 
betically, numerically, or directly with the distances they repre¬ 
sent. Once the scale is. adjusted you simply rack to any given 
point, refer to the scale, and you know at what distance you are 
photographing. 
Formula. 
Distance of 
Depth of 
Length of Image 
Scale Divided 
Dens from 
Field in 
when Object is 
into J inches. 
Object. inches at F/8 
6in. long. 
ft. in. 
Inches 
A — 
— 
..20 
2.5 
2.0 
R — 
— 
..2 3 
3.25 
1.87 
C 
— 
..2 6 
4.0 
1.75 
D — 
— 
..2 9 
4.5 
1.62 
E 
— 
..3 0 
5.0 
1.5 
F — 
— 
.•> 3 
6.5 
1.37 
G — 
— 
..3 6 
8.0 
1.25 
H — 
— ., 
..3 9 
0.5 
1.12 
I — 
— „ t 
..4 0 
10.25 
1.0 
J 
— 
..4 3 
11.0 
0.87 
K — 
— .. 
..46 
12.5 
0.75 
E — 
— . 
..4 9 
14.0 
0.62 
M — 
■— . . 
..5 0 
16.25 
0.5 
Every eighth of 
an inch racked 
produces three results. An 
alteration in the distance of the camera from the object, a change 
in the depth of field, and a larger or smaller image on the focus¬ 
sing screen, according to the direction of the adjustment, llius 
it will be found that eacli eighth inch racked one way or the 
other represents three inches nearer to or farther from the 
object. Whatever is taken off or added on with the scale, the 
image decreases or increases accordingly; while the depth of field 
widens or shrinks in accordance with the changed position. 
The figures of the formula are approximate, but sufficiently 
near for all practical uses, it being thought advisable not to en¬ 
cumber the reader with infinitely small fractions. Suppose that 
the photographer has located a nest with young, and the parent 
bird is six inches in length. On referring to the formula it 
will be found that the camera has to be set up at a distance of 
three feet three inches, to get a depth of field of about six and a 
half inches, while the image will appear about 1§ inches on the 
screen. If the focussing is done correctly, the bird may be 
photographed in any attitude required on the point in focus. 
Three-quarter breast on, back on, or profile may be secured, and 
thus a series will be obtained of the bird in every conceivable 
attitude. If, however, the bird does not alight or rest where 
you anticipate it to, a readjustment of focus must be brought 
about. As one-eighth inch racked on the scale this way or that 
reduces or increases the distance of the object three inches, there 
will be no trouble in making the necessary adjustment, without 
the bother of refocussing with the screen. 
