This section is from the book "Airplane Photography", by Herbert E. Ives. Also available from Amazon: Airplane photography.
Floor space in the cockpit being unavailable in the battle-plane, due to duplicate controls, bomb sights, etc., the English service was driven to the practice of carrying the camera in the compartment or bay behind the observer. Here it was attached either to the structural uprights or longerons, or to special uprights and cross-pieces built into the plane to serve photographic ends. As an intermediary between the camera and the supporting cross-pieces there was introduced the camera tray or cradle. This is essentially a frame carrying sponge rubber pads into which the camera is more or less deeply bedded. Figs. 83 and tests show this cradle to be superior to the outboard mounting, but still leave much to be desired. Its performance is better with the nose of the camera left free.
Fig. 82. - "L" camera in floor mounting.
 
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