A method and system for automatically calibrating a D.C. transducer are described in the environment of an illustrative film retrieval system. In this system, the output of a transducer such as a light sensor is amplified and the amplified output is compared to upper and lower limits within which it is desired to hold the transducer's amplified output. When either of the limits is exceeded, the amplification of the transducer's output is varied so as to shift its average value to a value which is between the upper and lower limits.