We must also find out the speed of the shutter required to take the
object in motion, so that it will appear as sharply defined as
possible under the circumstances. To do this the circle of confusion
must not exceed 1/100th of an inch in diameter. We therefore divide
the distance of the object by the focus of the lens multiplied by 100,
and then divide the rapidity of the object in inches per second by the
result obtained. This will give the longest exposure permissible in
the fraction of a second. For example, we require to know the speed of
a shutter required to photograph an express train travelling at the
rate of 50 miles per hour at a distance of 50 yards with an 8-1/2-inch
focus lens.
The train moves 876 inches per second.
1,800 distance in inches / (8-1/2 x 100) = 1,800 / 850 = 36/17.
876 speed of object per second / 36/17 = (876 x 17)/36 = 413
= 1/413 second.
Given the rapidity of the shutter, and the speed of the moving object,
we require to find the distance from the object the camera should be
placed to give a circle of confusion less than 1/100th of an inch.
Multiply 100 times the focus of the lens by the space through which
the object would pass during the exposure, and the result obtained
will be the nearest possible distance between the object and the
camera. For example, we have a shutter working at one-fiftieth of a
second, and the object to be photographed moves at the rate of 50
miles per hour. How near can a camera fitted with a lens of 8-1/2-inch
focus be placed to the moving object?
Object moving 50 miles per hour moves per second 876 inches, and in
the one-fiftieth part of a second it moves 17.52 inches, so that--
8-1/2 x 17.52 = 8.5 x 100 x 17.52 = 14,892 inches = 413 yards.
Instantaneous photography can only be successfully performed in very
bright and actinic light, and should never be attempted on dull days,
as underexposure will be the inevitable result. In developing it is
necessary to employ a strong developer to bring up the detail. Some
operators make use of an accelerator for this purpose, but it is not
to be recommended; the simplest is a few drops of hyposulphite
solution added to about 10 ounces of water. In this the plate is
bathed for a few seconds previous to development.
The following is a table by H. E. Tolman showing displacement on
ground glass of objects in motion:
==============================================================
| | Distance on | |
| |Ground Glass | |
| | in Inches | Same with | Same with
Miles per |Feet per |with Object 30|Object 60 Feet| Object 120
Hour. | Second. | Feet Away. | Away. | Feet Away.
----------+---------+--------------+--------------+-----------
1 | 1-1/2 | .29 | .15 | .073
2 | 3 | .59 | .29 | .147
3 | 4-1/2 | .88 | .44 | .220
4 | 6 | 1.17 | .59 | .293
5 | 7-1/2 | 1.47 | .73 | .367
6 | 9 | 1.76 | .88 | .440
7 | 10-1/2 | 2.05 | 1.03 | .513
8 | 12 | 2.35 | 1.17 | .587
9 | 13 | 2.64 | 1.32 | .660
10 | 14-1/2 | 2.93 | 1.47 | .733
11 | 16 | 3.23 | 1.61 | .807
12 | 17-1/2 | 3.52 | 1.76 | .880
13 | 19 | 3.81 | 1.91 | .953
14 | 20-1/2 | 4.11 | 2.05 | 1.027
15 | 22 | 4.40 | 2.20 | 1.100
20 | 29 | 5.87 | 2.93 | 1.467
25 | 37 | 7.33 | 3.67 | 1.833
30 | 44 | 8.80 | 4.40 | 2.200
35 | 51 | 10.27 | 5.13 | 2.567
40 | 59 | 11.73 | 5.97 | 2.933
45 | 66 | 13.20 | 6.60 | 3.300
50 | 73 | 14.67 | 7.33 | 3.667
55 | 80 | 16.13 | 8.06 | 4.033
60 | 88 | 17.60 | 8.80 | 4.400
75 | 110 | 22.00 | 11.00 | 5.500
100 | 117 | 29.33 | 14.67 | 7.333
125 | 183 | 36.67 | 18.33 | 9.167
150 | 220 | 44.00 | 22.00 | 11.000
----------+---------+--------------+--------------+-----------
[Illustration: FIG. 82.--ARTIFICIAL REPRODUCTION AND PHOTOGRAPHING OF
A MIRAGE]