PURPOSE: To change the magnification of a lens automatically, by sequentially repeating the measurement of center coordinate of B, G, R color signals and demagnetization, coinciding the color beams, and controlling the magnification of a zoom lens according to the degree of coincidence of the convergence.
CONSTITUTION: A cross hatch pattern generator 6 is switched to output B, G, R signals sequentially to obtain the center coordinate. A signal from a sequential signal processor 17 is processed at a controller 18, and the charging voltage and direction of current of a magnetizing power supply 25 are determined according to each center coordinate. A magnetizing current flows to a ring-shape magnetizing coil 24 with a charging signal from a controller 18 to move the coil so as to converge the beam. The detection of a center coordinate and charge/discharge are repeated and demagnetization is made so as to coincide the three color beams A pulley 14 fixed to a zoom lens 15' is turned with a pulse motor 42 so as to enter each center coordinate of each color signal into the whole effective section of a photo detector 26 to change the magnification of lens automatically.