In apparatus, such as a weapon sight, the speed of an object moving relative to a scene is determined by processing a plurality of successive frames of data, each obtained by scanning the scene, and detecting motion of the object relative to the background/foreground features of the scene. Each frame is compared with a reference comprising a different one of the frames and corresponding background features registered. Once the frames are registered, light intensity values of corresponding elements of the frames are subtracted. The registered background features, being stationary, cancel leaving the object features. Displacement of the object features over a predetermined number of frames is used to calculate the crossing speed of the object relative to the background features.