1. A method for monitoring the focus condition of a writing light beam impinging upon the writing surface of a recording medium, comprising the steps of: providing a source of high intensity light to form a writing light beam; directing said light beam through a signal-responsive light beam modulator; passing the modulated beam from the modulator along a path through a quarter-wave plate and an objective lens to impinge, in focussed condition, incident upon the writing surface of the medium; directing the light beam reflected from the medium in a reverse direction along said path through the objective lens, the converging lens, and the quarter-wave plate, and to the modulator; diverting the reflected light beam reaching the modulator out of the path of the incident writing beam and onto a light sensitive transducer; sensing the output signal of the transducer to monitor the focus condition of the writing beam at the surface of the medium; preserving a recording of the profile characteristics of the beam impinging the transducer when the writing beam impinging the surface of the medium is in focus, to define a reference profile; and when the writing beam impinging the surface of the recording medium is out of focus, adjusting the optics affecting focus of the writing beam on the recording medium until the beam impinging the transducer assumes said reference profile.