Coincidence rangefinder

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A Coincidence range finder (Stereoscopic range finder or Parallax range finder) is a type of range finder that uses mechanical and optical principles to allow an operator to determine the distance to a visible object.

The device consists of a long tube, with two lenses facing forwards at either end, and an operator eye pieces in the center. Two prism wedges which when aligned result in no deviation of the light are inserted into the light path of one of the two lenses. By rotating the prisms in opposite directions using a differential gear, a degree of horizontal displacement of the image can be achieved.

The Coincidence range finder uses a single eyepiece and uses a prism to merge images from both lenses into a single image to present to the operator. The operator adjusts the rotation of the prisms using a dial until the images overlap in the eyepiece. The degree of rotation of the prisms determines the range to the target by simple trigonometry.^[1]

A Stereoscopic range finder uses two eyepieces, and relies on the operators visual cortex to merge the two images into a single picture. A reference mark is separately inserted into each eye piece, the operator first adjusts the direction of the range finder so that the fixed mark is centered on the target, the prisms are then rotated until the mark appears to overlap in the operators combined view.^[2]^[3] Again the range to the target is proportional to the degree of rotation of the prisms.