This paper compares the performances of the binocular cues of stereo and vergence, and the monocular cue of focus for range estimation using an active vision system. The performance of each cue is characterized by its sensitivity to errors in the imaging parameters. The effect of random, quantization errors is expressed in terms of the standard deviation of the resulting depth error. The effect of systematic, calibration errors on estimation using each cue is also studied. Performance characterization of each cue is shown to be useful for active control of the imaging parameters to improve the accuracy of the estimated range. Also discussed, based on such characterization, are ways to integrate the use of the cues in order to overcome their individual limitations.