A friend was having trouble with underexposure on her Olympus OM-1 and brought her camera to me. The OM-1 was made to use the old 1.35 volt PX13/PX625 mercury cells which are now gone and most of the current replacement batteries are 1.5 volt alkaline cells. The battery in her camera was a 1.5 volt type which I tested and it was reading at its correct voltage. I used a modern Sekonic meter to test the camera's meter with this battery and then we tried it with new 1.35 volt Wein cell zinc/air battery and the difference between the two really surprised me. The difference between the alkaline and the Wein cell was 4 stops! The alkaline cell was producing under exposure by that amount. I tested it several times and it always came out as a 4-stop difference. I don't know if that's due to a peculiarity of the OM-1 meter but I now have a renewed respect for using Wein cells (or other voltage regulation means) in cameras that originally took mercury cells. Just thought I'd pass that along.