Abstract: A shutter phase lock circuit for a pair of biplane cinecameras having multiframe speeds utilized in a diagnostic X-ray system wherein the cameras are driven by polarized synchronous motors. Each of the cameras have a rotating shutter which is mechanically coupled to a commutator which has a 50% duty cycle and produces a square wave output pulse. The pulses are compared provided that the same camera speed has been selected for both cameras and in the event that two pulses are out of phase by a predetermined phase angle taking into account compensation for mechanical errors, causes an error signal pulse to be produced whose pulse width is a function of the camera speed selected.