1. A method for performing flexible pixel summing in an image sensor that includes a plurality of pixels grouped into subsets of pixels with each subset of pixels including three or more pixels operably connected to a separate common node, the method comprising: in a subset of pixels, selecting, in real time and based on one or more criterion, N pixels to include in a summing operation, where N represents a number that is between two and a total number of pixels in the subset of pixels; summing charge in the N pixels together by transferring the charge from the N pixels to the common node; and reading out the summed charge from the common node.