According to an example aspect of the present invention, there is provided an apparatus comprising at least one processing core, at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processing core, cause the apparatus at least to determine a reference point in a three-dimensional space based at least in part on locations of first and second features of a user's eye and on the user's gaze distance, and perform a mapping of the reference point into a viewed scene of a near-to-eye optical device to obtain an estimated gaze point and/or gaze direction of the user who is using the near-to-eye optical device, the mapping being based at least in part on calibration information associated with the user.