I recently purchased a good old pentax 645. It works really great, but I've been having trouble with the LED display inside the viewfinder showing up at times. When there is no film in the camera and I turn it on, the LED reads "f16 1000", which is normal. If I put a roll of film in it though it will give me a readout of whatever the meter reading is for the first few frames and then not show up at all. Once the film is out it works again. What's even stranger is that if I put a roll in with just the backing paper in (to test it), no film on it, it works fine. I've ran many rolls of film through this camera and it does this every time. I know that it is exposing properly when in automatic mode because my negatives are coming out wonderful. I'm using the 120 back with 120 film. any suggestions on why this may be happening?