In cameras where the lens can be moved to bring the object into focus on the film plane, there is usually a mechanism that indicates when the proper position of the lens has been reached. One system that is used for providing this indication is the optical rangefinder. A number of different versions of this device have been developed, but most of them are based on the design shown in the following figure. (The optical components that are shown in the figure are usually completely independent from the lens that is used to take the picture.)
The goal is to focus the camera on the object by observing its image on the viewfinder screen.
The user sees two images of the object on the screen inside of the camera. One image is produced by rays that travel along the direct path from the object to the screen as shown in purple. Rays traveling along this path strike the screen after passing through the fixed mirror, which transmits 50% of the light that is incident on it and reflects the other 50%. The second image is produced by rays traveling along the blue path. These rays are reflected twice before hitting the screen: once by a movable mirror that can rotate about an axis perpendicular to the figure and the second time by the fixed mirror. As the movable mirror is rotated, the blue image moves across the screen, and there is one position of the movable mirror for which the blue image exactly coincides with the purple image. (At other positions, the rays are reflected to other positions on the screen as shown by the orange rays, and the user sees a displaced image.) The position at which the two images overlap is determined by the geometry of the mirrors and by the distance of the object from the camera, so that the angle of the movable mirror can be calibrated to read the distance to the object directly.
The lever that rotates the movable mirror is coupled to the system that positions the lens, so that rotating the mirror until the two images overlap simultaneously adjusts the lens to focus on the object at that distance. This “coupled rangefinder” system is widely used on moderately-priced cameras.
The rangefinder is most sensitive to objects that are relatively near to the camera, and becomes less sensitive as the object moves further away because the corresponding changes in the angular position of the mirror become smaller and smaller at greater distances. This is usually not a serious problem, because the depth of field of the lens is also increasing as the object moves further away, so that the focusing becomes less critical at these distances.