1. An image sensor comprising: an array of pixels arranged in rows and columns, the columns being separated into groups each comprising a plurality of the columns and sharing a common output; and sampling and readout circuitry comprising, for each group of columns in the pixel array, a corresponding plurality of column circuits, the sampling and readout circuitry being configured to sample the common output for each group of columns independently into one of the plurality of column circuits associated with said group, and to read out the common output for each group of columns as previously sampled into another of the plurality of column circuits associated with said group.