Most points already covered. You are most probably using a Velvia type film which is very limited as far a latitude in exposure is concerned. When taking a reflective reading
be sure you are getting a reading from the highlight only, it is so easy to get some sky
or other zone in that will give you a false reading. Incidentally, I have found that the correct exposure for MY exposures is when velvia is rated at 40asa not 50. Stan. L-B