The Voigtlander 21/25mm Viewfinder (Black) is a shoe-mounted viewfinder that does double-duty, giving you a bright clear image, with brightlines marking a 21mm lens and a 25mm lens. This viewfinder has a lens with 5 lenses in 4 groups, magnification of x0.42, and a plastic body and a plastic foot.
Hotshoe-mounted viewfinder
Brightline markings for 21mm & 25mm lenses