An image sensor generates first digital samples and second digital samples during respective first and second sampling intervals, the first digital samples including at least one digital sample of each pixel of a first plurality of pixels, and the second digital samples including at least one digital sample of each pixel of a second plurality of pixels. A sum of the first digital samples is accumulated within a first counter as the first sampling interval transpires, and a sum of the second digital samples is accumulated within the first counter as the second sampling interval transpires.