It is constructed using a chrome plated brass mount plate and a combination of aluminum and composite lens barrels to reduce weight while maintaining strength and durability. 242 uses high quality optical glass multicoated lens elements. Two Aspherical lens elements and one SD (super low dispersion) glass element are employed in the optical design to insure sharpness across the film plane. The aspherical elements are each molded as one piece of optical glass rather than a resin laminated to a glass core. An internal focusing mechanism gives this lens fast auto focus. Focus distance is indicated on the manual focus ring. Like the rest of the lenses in the AT-X series, The AT-X 242 AF is what is commonly referred to as a parafocal lens, meaning when focus is achieved at a telephoto focal length, it is possible to zoom back to a wide angle and maintain focus on the subject.