I would like to test the shutter speeds on my F4s. I cannot get the shutter to fire with the back open. I'm sure there's a release button some where on the open back, but I can't seem to locate it. Any help?
Open the back, look at the groove where the seal is supposed to be and right underneath the rewind crank. There is a little silver button there. There is another one on the other side but you only need to push the one on the rewind crank side and the shutter works.
At a swap meet there was a guide who tested shutter speed and he used break a piece of a toothpick and jam it in there.