The dependence of perceived contrast on stimulus contrast of sinewave gratings was measured by the method of magnitude estimation. The resulting perceived contrast functions are well described by threshold-corrected power functions with exponents near 0.7. The exponents are insensitive to changes in mean luminance from 10-340 cd/m2 and to changes in spatial frequency from 0.25 to 12 c/deg. The exponents are also insensitive to a change in the range of grating contrasts from 1-2 log units. However, the distribution of contrast levels within the range produces small, but predictable, effects. Several factors are identified that may account for discrepancies in previous measurements of perceived contrast functions.