III
THE SUSPENSION AND INSTALLATION OF AIRPLANE CAMERAS
CHAPTER XIV
THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION
General Theory.—In addition to the limitation of exposure set by the ground speed of the plane another limitation is set by the vibration of the camera. This may be caused either by the motor, or by the elastic reactions of the plane members to the strains of flight. Unlike the movement of the image due to the simple motion of the plane, movements due to vibration may be eliminated by proper anti-vibrational mounting of the camera.
The effect of vibration may show as an indistinctness of the whole image—this is its only effect with a between-the-lens shutter—or as a band or bands of indistinctness parallel to the curtain opening (Fig. [76]). These are due to shocks or short period vibrations during the passage of the focal-plane shutter.
The obvious remedy for vibration troubles is to mount the camera on some elastic, heavily damping support, like sponge rubber or metal springs. Such a mounting should, however, be designed on sound principles derived from a proper analysis of the nature and effect of the possible motions of the camera. Otherwise, the vibrational disturbances may be increased rather than diminished by the camera mount. Such an analysis, based merely on general mechanical principles, shows that all motions of the camera are resolvable into six. These are three translational motions, namely, two at right angles in one plane such as the horizontal, and one in the plane at right angles to this (vertical); and three rotational motions, one about each of the above directions of translational motion as an axis (Fig. [77]).
Fig. 76.—Captured German photograph, showing zones of poor definition due to vibration during passage of focal plane shutter aperture.
Brief consideration will show that only the latter—the rotational motions—are of any importance when the small displacements due to vibration are in question. To illustrate the negligible effect of vibrations which merely move the camera parallel to itself in any direction it is only necessary to imagine the camera moved parallel to the ground through a large distance, such as 10 centimeters. Now 10 centimeters motion of the camera at 3000 meters elevation means, with a 25 centimeter camera lens,
| .25 | 1 | ||
| × 10 = | centimeter | ||
| 3000 | 1200 |
motion on the plate, which would be only a tenth the distance separable by a good lens. If we reduce this motion to the small fraction of a centimeter which vibration would actually produce, it is evident that such vibration is of absolutely no importance. Similarly, if we imagine the camera, under the same conditions, moved vertically with reference to the ground by ten centimeters, the scale of the picture would merely be changed by 1
12000 or by 1
1000 centimeter on a 12 centimeter plate, again quite negligible.