I recently bought a Mamiya 55mm f2.8 N/L leaf shutter lens for my 645 system. I also obtained a photocopied set of instructions for the leaf shutter lenses. As those who own one would know, when the lens is at rest with the shutter set on any position from 1/30 to 1/500 the shutter is in a closed position, but if the shutter setting is in the "F" (for focal plane) position, the shutter is fixed in an open position until it is later set to one of the other settings and fired. My question is this: is it detrimental to leave the lens for long periods with the shutter in the open "F" position, or does it not make any difference? I have not seen any mention of this in the instructions.