A user (18) views a reference symbol (36) along a line of sight (28) intersected by an image intensifier (22), arranged to present an intensified image of a forward scene (26) to the eye (24) of the user (18), and an image display (14), arranged to display an image to the eye (24) of the user (18) overlaid on the intensified image. The user indicates alignment of first and alternate symbols generated at known positions on the image display (14) with the reference symbol (36) and the position of a helmet (16) carrying the image display (14) is determined by a helmet tracking system (30) for each alignment. A line of sight can be determined for each symbol alignment given the helmet (16) position and known location of the first and alternate symbols. These lines of sight can be used to determine the magnification factor of the forward scene (26) induced by the image intensifier (20). Accordingly, symbology generated and displayed on the image display (14) can be compensated for relative to the magnification factor.