The paper describes image correlation in a photorefractive based nondegenerate four-wave mixing system. The system is arranged such that the reference image may be changed arbitrarily quickly, and work is described which measures the rate at which the input image can be updated. It is shown that a photorefractive correlator may work at the maximum rate allowed by a TV system; i.e. the grating can be written during the flyback time of the TV camera. TV framelocked operation is described. It is shown that the correlator may provide a true correlation output or a weighted correlation, depending on the relative intensity of the write beams. Numerical simulation of a theoretical model of the correlation process supports the experimental results.