An encoder apparatus including a scale and a readhead including a sensor including an array of sensor elements for detecting a scale signal falling on the sensor, configured such that the sensor elements' outputs that are used to determine the relative position of the scale and readhead are weighted such that the influence of the sensor elements on the determined relative position of the scale and readhead varies across the array according to a predetermined non-rectangular window function that is configured to reduce the adverse effect of undesirable frequencies in the scale signal so as to thereby reduce the encoder apparatus' sub-divisional error.