Apparatus for controlling the focus of a camera arranged to capture a sequence of frames, includes an image processor configured to: form an image characteristic for a plurality of blocks of a first frame, each block comprising one or more pixels of the first frame; and calculate an image parameter for each block by combining the image characteristics of blocks lying within a predefined zone relative to that block; and a focus controller configured to derive a measure of focus for a selected frame area of the first frame by identifying a set of blocks whose respective predefined zones, when combined, substantially represent the selected frame area, and forming a measure of focus for the selected frame area by so combining the image parameters of the set of blocks; wherein the focus controller is configured to generate a signal for controlling camera focus in dependence on the measure of focus formed for the selected frame area of the first frame.