A method in an eye tracking system for providing an approximate gaze convergence distance of a user comprising: determining calibration data in relation to interpupillary distance (x) between a pupil of a left eye and a pupil of a right eye of a user; determining, based on the determined calibration data, a gaze convergence function (f(x)) providing an approximate gaze convergence distance of the user based on a determined interpupillary distance of the user; receiving, from one or more imaging devices, one or more images of the left eye and the right eye of the user; determining a current interpupillary distance of the user based on the one or more images; and determining a current approximate gaze convergence distance based on the current interpupillary distance and the gaze convergence function, characterized in that: the method further comprises capturing the one or more images using the one or more imaging devices and one or more holographic mirrors, each holographic mirror having a reflection angle varying across its surface, such that with a predefined position of a corresponding imaging device of the one or more imaging devices, the magnification of a representation of an object in an image captured is constant over distance of the user's eyes to the corresponding imaging device; and in that the calibration data comprise an approximation of the eye ball diameter of the user's eyes. determining calibration data in relation to interpupillary distance (x) between a pupil of a left eye and a pupil of a right eye of a user; determining, based on the determined calibration data, a gaze convergence function (f(x)) providing an approximate gaze convergence distance of the user based on a determined interpupillary distance of the user; receiving, from one or more imaging devices, one or more images of the left eye and the right eye of the user; determining a current interpupillary distance of the user based on the one or more images; and determining a current approximate gaze convergence distance based on the current interpupillary distance and the gaze convergence function, characterized in that: the method further comprises capturing the one or more images using the one or more imaging devices and one or more holographic mirrors, each holographic mirror having a reflection angle varying across its surface, such that with a predefined position of a corresponding imaging device of the one or more imaging devices, the magnification of a representation of an object in an image captured is constant over distance of the user's eyes to the corresponding imaging device; and in that the calibration data comprise an approximation of the eye ball diameter of the user's eyes. the method further comprises capturing the one or more images using the one or more imaging devices and one or more holographic mirrors, each holographic mirror having a reflection angle varying across its surface, such that with a predefined position of a corresponding imaging device of the one or more imaging devices, the magnification of a representation of an object in an image captured is constant over distance of the user's eyes to the corresponding imaging device; and in that the calibration data comprise an approximation of the eye ball diameter of the user's eyes.