. A lens situated at or near the plane of an internal image to project the aperture of a previous objective or erector upon the aperture of a following lens. 2
. A lens at or near the plane of a ground-glass screen to bend the edge light toward the eye of an observer; a Fresnel lens often is used in this capacity. 3
. That component of an eyepiece closest to the objective; it determines the diameter of the field of view of the instrument.