1. An apparatus for controlling the position of a screen pointer, the apparatus comprising: an at least partially coherent light source for illuminating an imaging surface, thereby generating reflected images; and a navigation sensor for generating digital images based on the reflected images, performing a movement computation based on the digital images, and generating movement data based on the movement computation that is indicative of relative motion between the imaging surface and the apparatus, wherein the movement computation comprises summing only pixels values from a first one of the digital images, thereby generating a first plurality of sums, and summing only pixel values from a second one of the digital images, thereby generating a second plurality of sums.