(32) 
a tri-camera mount are suspended by cotton sash cord. The simple single 
camera mount, with drift and levelling adjustment, is shown in Fig. 10. The 
vibration isolation characteristics of a number of types of mounts have been 
checked by flying at night with the camera shutter opened over a special arrange 
ment of synchronized flashing lights. Angular vibration characteristics about 
three perpendicular axes are determined from the photographs 19 . A mount 
is considered to be satisfactory if the maximum relative motion between the 
image and the film arising from angular motion during the longest exposure does 
not exceed one third of the average resolved distance for the lens-film combination 
as determined with a test object having a density difference of 0-2. 
Fig. 10 Mount for Eagle V camera 
All aerial camera installations use a camera port glass. At any angle up 
to 45° from the normal this window is required to cause no greater deviation of 
rays passing through it than 30 seconds of arc nor any rate of change of deviation 
greater than 10 seconds per inch from place to place over its area. 
Practically all photography is done only in clear weather. The poorest 
conditions normally encountered are those due to smoke, haze, and mountain 
shadows. A very thin cirrus overcast is tolerable for large scale forestry photo 
graphy intended for cutover delineation only. For log pile volume determin 
ations where equal illumination on both sides of the pile is considered desirable, 
photography has been carried out from 1,200 ft. under solid, low overcast at 
mid-day. Exposure is determined from the camera operator’s experience.