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.
FIG. 81.—"A RISE IN THE WORLD."
BY THE MARQUIS DE ALFARRAS.
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—