An optical system (10) for computing a height of a target (22) on a surface includes a light projector (30) for projecting light. The light passes through a patterned reticle (32) and a projector lens (34) so as to illuminate the target (22) with an image of the pattern. The light is projected telecentrically between the reticle (32) and the projector lens (30), and a camera (42) is positioned along a receive optical path. The camera (42) receives an image of the target (22) through a receive lens (40). The target (22) and the pattern move at least three times with respect to each other, and the camera (42) acquires an image of the target (22) at each of at least three positions.