A rangefinder is an optical device used to determine the distance to a subject for accurate focusing, either built into older cameras or attached as an external accessory. It works by presenting two slightly different viewpoints of the scene, creating a double image. The photographer adjusts the lenses until the two images align into one, providing a precise distance reading that can then be transferred to the cameras focusing mechanism.
Rangefinders were especially popular in film and early digital cameras for street, portrait, and architectural photography, offering fast, accurate focusing without relying on the cameras viewfinder or autofocus system. Their precision and simplicity make them a valuable tool for controlled, manual photography.