Abstract: An automatic focussing arrangement, incorporated in a system in which an optical image is focussed onto a plane, comprises a sensor assembly located in the region of the plane to sense the image. The sensor assembly comprises at least two arrays of sensor devices, one array of sensor devices being in front of the plane, and the second array of sensor devices being behind the plane. Means are provided for moving the image relative to the sensor assembly or for moving the sensor assembly relative to the image. From the first and second arrays signals are derived representative of the image received by the said sensor devices. A focus drive signal to maintain the focus of the arrangement as desired is derived from the signals.