A rangefinder is a device used to determine the distance between one object and another. It works along the same basic principle as the telescope, and often features etchings engraved into the lens which make up the rangefinding element. However, laser rangefinding technology has taken over from manual rangefinders in most everyday uses, and nowadays only marksmen and photographers tend to use the manual kind.
Coincidence rangefinders were developed throughout the First and Second World Wars for use on naval vessels, predominantly; during WWII they were also used by snipers. Naval rangefinders were often some 6-10 feet in length, and were manned by a crew consisting of several sailors.
Inside the tube two prism wedges are maneuvered into (or out of) alignment by means of a differential gear, which moves the image along the horizontal axis. The device uses a single eyepiece to merge the images from both lenses into one single composite image, and then present it to the operator. By moving the dial (and thus adjusting the prisms) until the two images converge and overlap in the eyepiece. The angle of the prisms can then be read, and the distance to target can then be read using simple trigonometry.
A Stereoscopic range finder works in a similar way, but uses two eyepieces, and takes advantage of the human eyes’ own ability to focus two pictures into one single image in the same way that binoculars work.. A reference point is inserted or engraved into each eye piece, and the operator lines up the direction and orientation of the range finder until the mark is overlaid upon the target. The prisms are then rotated until the two individual images merge seamlessly, and then the range is found by noting the degree of rotation required to fix the two images.
Rifle rangefinding scopes work by using a series of lines or concentric circles, each line corresponding to a set distance, usually an increment of 100m. By placing a certain point of the target in the centre of the scope’s view, a marksman can use the crosshairs and the rangefinding lines to determine the distance to the target. This principle has remained fundamentally unchanged since it was first developed during the First World War, and has the distinct advantage of not generating any light, thus not giving away the snipers’ position.
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