Oh, horrors! Many years ago I conducted a test with my OM-1n (I am the original purchaser), with mercury oxide cell and with alkaline cell. There was definitely an error in the meter reading when I used alkaline 1.5v battery rather than the 1.35v mercury oxide cell. The amount of metering error is NOT constant and cannot be simply compensated with adjustment of ISO value. The amount of metering error depends not only upon the precise voltage being out out by the alkaline chemistry (which varies as the cell depletes), but also based upon the amount of light in the scene being metered!!!
Originally Posted by paul ron