A system comprises a display object having a characteristic of its display configured to change with time in a timing calibration operation; a plurality of video cameras configured to capture images of the display object; and a data processing apparatus comprising a detector configured to detect timing differences in the capture of images by the plurality of video cameras indicative of a change in the characteristic of the display object and to apply the detected timing differences as a timing correction so as to time-align images captured by the plurality of cameras after the timing calibration operation.