An experiment was conducted to investigate the effects of image size range, image polarity, and modulation transfer function MTF on discrimination of warship targets with an electro-optical sensor. The imagery, which consisted of static warship silhouettes, was presented to subjects on a TV monitor in a controlled, laboratory environment. Image size was varied so that the target image lengths took on values of 15, 20, and 30 of the display width. MTF was varied over a relatively small range, with the best curve yielding a response of 0.5 at a spatial frequency of 15 cycles per display width and the poorest curve yielding only 0.23 at the same spatial frequency. Changes in image size caused the strongest effect, with performance at the largest image size averaging 57 higher than for the smallest image size. Correct identifications averaged 40 higher with the best MTF curve than with the worst. Performance against light targets on a gray background averaged 15 higher than for dark targets on the same background.