Gian Paolo, Since, the original mercury oxide batteries have been discontinued, I'll hazard a guess, you are used the newer slightly higher voltage batteries. (sorry, I can't remember the designation numbers) You can have your camera's meter adapted for the new batteries, or get an aadapted/converter. However, if it is a consistent 1/3 of a stop difference, why bother? Just leave the camera at 80 iso. Happy Snaps, Sal