In a method of converting on optical signal into an electronic video signal by means of cathode ray tube ('CRT') scanning means (42), each image is scanned in a plurality of scanning operations with each scanning operation using a different area (32,34) of the screen of the CRT scanning means and the signals resulting from the plurality of scanning operations are combined to provide the electronic video signal. Light from the CRT scanning means is passed through deflection means (48) operable to deflect light from respective ones of the plurality of different areas (32,34) of the screen of the CRT scanning means (42) along the same path into an objective lens so that the same area of a film gate positioned in the optical path beyond the objective lens is scanned in each scanning operation. The output of the CRT scanning means (42) is monitored by means of a photoelectric device operable to provide an output signal when a blemish is detected on the screen of the CRT scanning means. The output signal is applied to an analogue-to-digital converter used to process the electronic video signal to cause the video signal level immediately prior to detection of the blemish to be maintained while the blemish persists.