G02B23/00—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices
G02B23/14—Viewfinders
G—PHYSICS
G02—OPTICS
G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
G02B17/00—Systems with reflecting surfaces, with or without refracting elements
G02B17/02—Catoptric systems, e.g. image erecting and reversing system