This Leica Brightline Finder M-21 is a viewfinder for 21mm M lenses. The finder provides a remarkably vibrant, high-contrast image. Due to a concave mirror in the optics, part of the incoming light is used to illuminate the picture outline as a bright frame. This is the same as in viewfinders of Leica M cameras.

The finder has parallax compensation markings for distances below 6.56' (2m) and also has markings for the slightly smaller image field of Leica M8 models. It can therefore be used for film and digital photography.

The optics are given a high-quality multi-layer coating to guarantee optimum viewing through the finder in all conditions. The high-quality metal body of the finder is designed in the popular retro look of the fifties and sixties, but is milled from solid brass according to the latest technologies and then given a silver-chrome finish. The eyepiece side is rubberized to protect eyeglasses.