1. A system for encoding information from the optical image of a camera, comprising, (a) means for representing a predetermined characteristic of said image by an array of values, (b) means for sampling an ordered sequence of the values of said array, and for marking transitions in said ordered sequence by a sequence of marker values, and (c) means for autocorrelating said sequence of marker values to provide an autocorrelation function for said sequence of marker values, and thus for said ordered sequence.