I would concur with E. Clean the battery contacts and also the prism contacts and always carry a spare battery. Even then I find there are pretty large variations from one finder to the next. At one point I had three of them so I close inspected/tested all of them and ended up selling the two least accurate ones, even they were all within 1 or 2 stops of each other. The one I ended up keeping was actually the worst looking of the lot but was the most accurate.