Fig. 18.—50 centimeter F/6 lens in U. S. standard mount, showing color filter retaining ring and catch.
A somewhat different and better method of tightening the lens in the flange, when focussed, has been adopted in the English lens mount, which is in general similar to the American. The threaded part of the flange is split by a slot cut parallel to the flange base, and a screw is run into the flange from the front, through the split portion. By tightening this screw, which is always accessible, the split part of the flange is squeezed together, thus rigidly holding the lens barrel.
CHAPTER V
THE SHUTTER
Permissible Exposure in Airplane Photography.—A definite limitation to the length of exposure in airplane cameras is set by the motion of the plane. If we represent the speed of the plane by S, the altitude of the plane by A, and the focal length of the lens by F, we obtain at once from the diagram (Fig. [19]), that s, the rate of movement of the image on the plate, is given by the relation,
| s | F | |
| = | ||
| S | A |
If we call the permissible movement d, then the permissible exposure time, t, is given by the relation—
| d | Ad | ||
| t = | = | ||
| s | FS |
As a representative numerical case, expressing all quantities in centimeters and in centimeters per second, let F = 50, S = 20,000,000
3600 (200 kilometers per hour), and A = 300,000, then
| 50 × 20,000,000 | ||
| s = | = .9 centimeters | |
| 300,000 × 3600 |
If we take for the permissible undetectable movement, .01 centimeter, which is, as has been shown, a reasonable figure for lens defining power, we have, then, that the longest permissible exposure is .011 second—in round numbers, one-hundredth.