OBJECTIVE To describe lens and perspective distortion using new measures that have practical meaning to the surgeon, and to apply these measures to show the extent of optical distortion in rigid endoscopes used in endoscopic sinus surgery. METHODS Laboratory measurements on rigid endoscopes. METHODS Barrel and perspective distortion were measured in 4-mm diameter 0°, 30°, 45°, and 70° rigid sinus endoscopes. Images of square grids were obtained with the endoscopes aligned in a specially constructed test rig. The terms relative size (RS) and relative distance (RD) were introduced to describe size and distance errors; and the term relative angle (RA) was used for assessing perspective errors. RESULTS All the endoscopes exhibited similar barrel distortion. RS of the image at the periphery was 52%; RD was 80%. For RA values of 30°, 45°, and 70°, RS values were 77%, 58%, and 32%, respectively. Objects at the edge of the surgical field appear significantly more distant than suggested by their screen position. Perspective distortion occurs, unless RA = 0°. Barrel distortion of the lens helped to offset the effects of perspective distortion. CONCLUSIONS Optical distortion can be quantified and understood using straightforward definitions. High levels of distortion are common, particularly due to perspective distortion, which is dependent on RA but independent of barrel distortion and the viewing angle of the endoscope.