An optical device for effectively adjusting the F-number of an elliptical lamp is provided, the elliptical lamp for producing a focused light beam at a given focal point having a given cone angle. The optical device comprises a light interaction portion for optically interacting with the focused light beam when the light interaction portion is in general longitudinal alignment with a light emitting aperture of the elliptical lamp, the light interaction portion for triggering optical adjustment of a cone angle of at least a high cone angle portion of the focused light beam to a smaller cone angle. The optical device further comprises a light egress portion, coupled to the light interaction portion, for enabling exit of the focused light beam from the optical device with an effective cone angle smaller than the given cone angle, after the cone angle of the at least the high cone angle portion of the focused light beam has been adjusted to the smaller cone angle.