An optical rangefinder is a distance-measuring device, used in photography as a focusing aid. Cameras equipped with this device are called rangefinder cameras. There are two types of optical rangefinder: a coupled rangefinder has a linkage to the distance ring of the lens and lets you focus on a subject directly with the lens's focusing ring, while an uncoupled rangefinder allows you to measure how far away you are from a subject, and then manually transfer the measurement to the camera's actual distance ring. Accessory uncoupled rangefinders may be used with almost any camera with a focusing lens.
Split-image devices, often built in to SLR focusing screens, may also be referred to as rangefinders. There are other distance measuring focusing devices used on autofocus cameras - using infra-red light or, occasionally, sonar; these are not usually termed rangefinders.
The typical rangefinder is known as a coincident image type, and displays two separate images. The main image is the entire frame of the viewfinder; while the secondary image is a small spot in the center of the frame. To focus on something, you aim the spot in the center at that subject, which shows the two images. By turning the focus adjustment, you move the secondary image horizontally until it lines up with the main image. At this point, the rangefinder (and so the lens, if coupled) will be focused on the selected object.
In the example (right), the light image area in the middle of the viewfinder shows a double image of the antenna. This indicates that the lens focus is not correct for that distance. By adjusting the focus control (typically a lens ring), the two images are brought into alignment; at that setting the antenna will be sharply focused.
Some newer electronic cameras - such as the Contax G2 use a form of rangefinder as part of their autofocus mechanism.
The rangefinder works by combining two images, from two windows, into the image you see, using a beamsplitter. This is a "half-silvered" prism or a mirror which can either split light, by reflecting some and allowing the rest to go straight through, or (in a rangefinder) used in reverse to combine views. The light from the second window is channelled via a mirror, prism or lens which rotates depending on the rangefinder setting - causing the secondary viewfinder image to move with the distance control.
Rangefinders are very accurate focusing devices over short distances - for camera rangefinders, typically up to about 10-15m (~30-40ft), actually more accurate than through-the lens focusing screens on SLRs. The accuracy depends on the effective rangefinder base - the distance between the viewing windows times the magnification of the viewfinder. Standard and wide-angle lenses are not very sensitive to focus from around 10m to infinity, so this limitation is not significant - unless using longer lenses. Rangefinders are not usually accurate enough for long, telephoto lenses - as these are more sensitive to focus at long distances, past where camera-sized rangefinders are effective.
| front of a coupled-rangefinder camera,|
with characteristic little extra viewfinder window
image by Martin Taylor (Image rights)
| Nettar with accessory rangefinder|
image by Romuald Swieconek (Image rights)
| Rangefinder accessory,|
subject side, scale on end
image by AWCam (Image rights)