Instantaneous Photography
In the very earliest days of photography this term was applied to what would now be considered very slow work indeed. We now usually apply this term when the exposure does not exceed one second. In some cases this only amounts to the one-thousandth part of a second. This exceedingly brief exposure is usually given to the plate by means of a suitably constructed shutter.
The immense strides that have recently been made in instantaneous photography, owing chiefly to the advent of the dry-plate process, have caused photography to become useful to almost every branch of science.
To Muybridge and Anschutz we are greatly indebted for the strides made in instantaneous photography. These gentlemen have succeeded in photographing moving objects hitherto considered impossible to be photographed. Galloping horses, swift-flying birds, and even bullets and cannon balls projected from guns have been successfully photographed, showing even the little head of air driven along in front of the bullet.
Both Muybridge and Anschutz also succeeded in making series of twenty-four or more photographs of a horse during the time it makes a single leap, and thus illustrated its every movement. The value of these and other possibilities with the camera for artists cannot be overestimated. Its aid to meteorologists in photographing the lightning, to astronomers in stellar, lunar and solar photography, and to all other sciences would require a work as large as this to describe.
For the making of instantaneous pictures a large number of suitable cameras have been devised. In most of these the lens is a very rapid one, and in some cases so arranged that all objects beyond a certain distance are in focus. With an instantaneous camera a secondary image is necessary, so that the right second can be judged for making the exposure. This is usually produced by a finder. In making instantaneous exposures the following tables may be useful:
Approximate distance
| A man walking 3 miles per hour moves | 4-1/2 | Feet per second |
| A man walking 4 miles per hour moves | 6 | Feet per second |
| A vessel traveling at 9 knots per hour moves | 15 | Feet per second |
| A vessel traveling at 12 knots per hour moves | 19 | Feet per second |
| A vessel traveling at 17 knots per hour moves | 28 | Feet per second |
| A torpedo boat traveling at 20 knots per hour moves | 35 | Feet per second |
| A trotting horse | 36 | Feet per second |
| A galloping horse (1,000 yards per minute) | 50 | Feet per second |
| An express train traveling at 38 miles an hour | 59 | Feet per second |
| Flight of a pigeon or falcon | 61 | Feet per second |
| Waves during a storm | 65 | Feet per second |
| Express train (60 miles an hour) | 88 | Feet per second |
| Flight of the swiftest birds | 294 | Feet per second |
| A cannon ball | 1,625 | Feet per second |
| An object moving— | ||
| 1 mile per hour | 1-1/2 | Feet per second |
| 2 miles per hour | 3 | Feet per second |
| 5 miles per hour | 7-1/2 | Feet per second |
| 6 miles per hour | 9 | Feet per second |
| 7 miles per hour | 10-1/2 | Feet per second |
| 8 miles per hour | 12 | Feet per second |
| 9 miles per hour | 13 | Feet per second |
| 10 miles per hour | 14-1/2 | Feet per second |
| 11 miles per hour | 15 | Feet per second |
| 12 miles per hour | 17-1/2 | Feet per second |
| 15 miles per hour | 22 | Feet per second |
| 20 miles per hour | 29 | Feet per second |
| 25 miles per hour | 37 | Feet per second |
| 30 miles per hour | 44 | Feet per second |
| 35 miles per hour | 51 | Feet per second |
| 40 miles per hour | 59 | Feet per second |
| 45 miles per hour | 66 | Feet per second |
| 50 miles per hour | 73 | Feet per second |
| 55 miles per hour | 80 | Feet per second |
| 60 miles per hour | 88 | Feet per second |
| 75 miles per hour | 110 | Feet per second |
| 100 miles per hour | 147 | Feet per second |
| 125 miles per hour | 183 | Feet per second |
| 150 miles per hour | 220 | Feet per second |
| 200 miles per hour | 257 | Feet per second |
With these tables it will be very easy to find the distance that the image of the object will move on the ground-glass screen of the camera. To do this, multiply the focus of the lens in inches by the distance moved by the object in the second, and divide the result by the distance of the object in inches.
Example, find the movement of the image of an object moving 50 miles per hour at a distance of 100 yards with a lens of 9-inch focus.
9 × 876 = 7,884 ÷ 3,600 = 2-1/5 inches per second.
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 × 100) = 1,800 ÷ 850 = 36/17.
876 speed of object per second ÷ 36/17 = (876 × 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 × 17.52 = 8.5 × 100 × 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:
| Miles per Hour. | Feet per Second. | Distance on Ground Glass in Inches with Object 30 Feet Away. | Same with Object 60 Feet Away. | Same with Object 120 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 |