An auto exposure method for an image sensor includes (a) evaluating variance, for each of a plurality of histograms of the pixel values from a respective plurality of individual exposures of the image sensor at respective exposure 5 time settings, of contribution from individual bins of the histogram to total entropy of the histogram, to determine an optimal exposure time for the image sensor corresponding to a minimum value of the variance, and (b) outputting the optimal exposure time to the image sensor.