A system for automatically registering and tracking at least one eye (101; 102) of at least one subject, comprising: an optical system including an image sensor adapted to register spatially distributed light and produce resulting primary data (D S), and a lens structure (111, 112) adapted to receive incoming light reflected from a scene containing the at least one subject and direct at least a portion of the incoming light towards the image sensor, the optical system having a basic optical transfer function, and a processing unit adapted to derive eye-tracking data (D EYE) based on the primary data (D S), wherein the optical system further comprises a mask which is arranged between the at least one subject and the image sensor, the mask is adapted to alter the basic optical transfer function into an enhanced optical transfer function which is adapted to spread an image from one point in space onto more than one sensor element on a sensor surface of the image sensor, and the enhanced optical transfer function is substantially less sensitive to variations (Δ) of an unknown distance between the optical system and the at least one subject than the basic optical transfer function, the processing unit is adapted to receive the primary data (D S), and process the primary data (D S) to produce resulting eye-tracking data (D EYE) representing at least one of a position estimate of the at least one eye (101; 102) and a gaze direction for the at least one eye (101; 102), and the image sensor is adapted to exclusively transfer a high-relevance fraction of the primary data (D S) to the processing unit, the high-relevance fraction being selected by the processing unit based on at least one of previously derived eye-tracking data (D EYE) and data registered by one or more auxiliary sensors connected to the system, characterized in that the processing unit is further adapted to match the primary data (Ds) against a target shape representing a typical eye shape transformed by the enhanced optical transfer function. an optical system including an image sensor adapted to register spatially distributed light and produce resulting primary data (D S), and a lens structure (111, 112) adapted to receive incoming light reflected from a scene containing the at least one subject and direct at least a portion of the incoming light towards the image sensor, the optical system having a basic optical transfer function, and a processing unit adapted to derive eye-tracking data (D EYE) based on the primary data (D S), wherein the optical system further comprises a mask which is arranged between the at least one subject and the image sensor, the mask is adapted to alter the basic optical transfer function into an enhanced optical transfer function which is adapted to spread an image from one point in space onto more than one sensor element on a sensor surface of the image sensor, and the enhanced optical transfer function is substantially less sensitive to variations (Δ) of an unknown distance between the optical system and the at least one subject than the basic optical transfer function, the processing unit is adapted to receive the primary data (D S), and process the primary data (D S) to produce resulting eye-tracking data (D EYE) representing at least one of a position estimate of the at least one eye (101; 102) and a gaze direction for the at least one eye (101; 102), and the image sensor is adapted to exclusively transfer a high-relevance fraction of the primary data (D S) to the processing unit, the high-relevance fraction being selected by the processing unit based on at least one of previously derived eye-tracking data (D EYE) and data registered by one or more auxiliary sensors connected to the system, characterized in that the processing unit is further adapted to match the primary data (Ds) against a target shape representing a typical eye shape transformed by the enhanced optical transfer function.