Rangefinder Design - The Rotating Wedge

Viewfinders & Rangefinders > Rangefinder Design - The Rotating Wedge

The rotating wedge design is an improved rangefinder design that uses two cylindrical elements to bend light. The cylindrical elements are placed with a small air space (negligible), and the rear element is rotated around the center of its radius. This design offers gives a displacement angle 2 times greater than the focus angle, or 4 times that of the rotating mirror design. A more in depth explanation of the maths is available in Josef Stüper’s book, Die Fotographische Kamera. Although a fair warning, it is in German. The design also appears in Norman Goldberg’s book Camera Technology, The Dark Side of the Lens p24-26. Although in broad strokes.

From left to right: zero position, closest focus, adjusted zero position to maximize glass use

To optimize the design, the elements can be set so that when deviating the light by α/2 it produces the correct path, this requires the elements to be offset by tan α · d , where d is the distance between the rear piece and the front surface mirror. The mirror itself also has to be offset by α/4. This can be explained simply through the fact that the beam that is reflecting off of it has to become perpendicular, looking at the rotating mirror design, when the beam has an angle α, the mirror has to be offset by α/2.

Cameras that use the design : Contax II, Kiev 3, 4 etc.., Zeiss Ikon Finder for Polaroid. - help expand the list.

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