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film counter

Some conspiracy people mention that there was no way of telling if a picture was successfully taken as the frame number can hardly be seen through a small window on the side of the camera body. After using such a camera system for many years, the vibration and sound from the shutter opening and closing is enough to know the shot was indeed taken. Sound can travel from the camera and into a spacesuit; sound travels through all solids, even in a vacuum.

The counter was there to tell the photographer how many frames were left, not to tell you that a picture was taken. In the case of Apollo, bulk film backs were used. As long as he didn't take any more than 160 shots while on the surface, the was no need to worry about it and change the film back. It would be changed when back inside the Lunar module. 

More information in the book.