There is no fixed speed at which a photographic shutter should be worked, because so much depends upon the strength of the light, the aperture of the lens, the speed of the plate, and the rapidity with which the objects it is desired to photograph are moving. The exposure will generally be as long as the moving objects will allow. When the distance from the camera to the moving object and the speed at which it travels are known, an excellent rule is as follows: - Divide the distance between the camera and object (in inches) by the focus of the lens multiplied by 100, and divide the result by the rapidity of motion (in inches) to obtain the answer in the fraction of a second. Thus, if the distance of object is 720in., the focus of lens 7in., the rapidity of motion 20 miles an hour or 352in. per second; then x 720/(700x352) = 1/340 of a second, which is the speed at which the 7shutter must be worked to obtain a sharp image, assuming that the greatest amount of blur or confusion admissible in any point of light must not exceed 1/100 part of an inch. It then only remains to find what lens aperture and plate will allow of so brief an exposure being given on such a subject and in such a light.

For example, if f/8 at 12 noon in June requires 1/150 of a second to secure desired density of negative, etc., then f/5.6 will be the nearest stop to give the correct result at the same time.