A rangefinder is a focusing mechanism that measures the distance to a subject by triangulation, and by extension the name for a camera that uses one. Looking through the viewfinder you see a small bright patch in the center showing a second, overlaid image of the scene. You turn the focus ring until the two images merge into one, and at that point the lens is focused on that subject.
The system works by combining the views from two windows set a short distance apart on the camera body. As you focus, a coupled mechanism rotates the angle of the second view, and when the doubled image aligns the geometry of that angle corresponds to the subject distance. The distance between those two windows, called the base length, sets how precisely the rangefinder can focus, which is why these cameras handle fast wide and normal lenses superbly but struggle to focus long, fast telephotos accurately.
Because focusing does not happen through the taking lens, a rangefinder offers a different way of seeing than the through-the-lens view of an optical or electronic viewfinder on an SLR. The bright finder shows area outside the frame lines, so you can watch a subject enter the frame, which many street and documentary photographers prize for anticipating a moment.
That separate viewfinder is also the format’s main limitation. Because the finder sits beside the lens rather than behind it, close subjects suffer from parallax error, corrected by moving frame lines but never perfectly, and the system cannot do macro or long telephoto work where through-the-lens viewing is essential. The finder also shows the same view regardless of the lens, with frame lines marking the coverage of each focal length.
Rangefinders reward a deliberate, manual focus way of working, and they suit a technique called zone focusing, where you preset the focus distance and aperture so anything within a range is sharp, letting you shoot instantly without focusing at all. They are typically compact and quiet, with no mirror to slap, and pair beautifully with wide and normal lenses. The Leica M series is the archetype, in both film and digital form, and remains the reference point for the format.
Compared with the DSLR and the modern mirrorless camera, a rangefinder trades versatility for a focused, unobtrusive shooting experience. It is a niche choice today, but a devoted one, especially among photographers who value the discipline and the bright, lag-free window on the world it provides.
For a newcomer curious about the format, the honest advice is to try one before committing, since the focusing method and the separate finder suit some photographers perfectly and frustrate others. Those it suits tend to describe a slower, more considered way of shooting that they find hard to give up, even as autofocus cameras race ahead on every spec sheet.