Utilising focussing of light sources in image for controller tracking

A controller-tracking system includes: camera(s) arranged on head-mounted display (HMD); one or more light sources arranged on controller(s) to be tracked, and controller(s) being associated with the HMD. The light sources provide light having wavelength(s). The processor(s) are configured to receive image(s) representing controller(s); identify image segment(s) in image(s) that represents light source(s); determine level of focussing of light source(s) in image(s), based on characteristics associated with pixels of image segment(s); determine distance between camera(s) and light source(s), based on level of focussing, intrinsic parameters of camera(s), and reference focussing distance corresponding to wavelength of light provided by light source(s); and determine pose of controller(s) in global coordinate space of real-world environment, using distance between camera(s) and light source(s).