How a Leica M-style rangefinder works: what happens to the light, step by step, and every part that does it.
An illustration of the classic rangefinder layout, not any one camera's construction. Seen from behind, as you hold it, unless a drawing says otherwise.
the light's path, through the lens to the filmglasswhat moves when you focus
Drag across the drawing to take it apart · tap a part to see it on its own
How it works
1
The lens brings the light to a point
The lens is a stack of glass, here six elements in four groups. Each curved surface bends the light a little, so the light that spreads out from one point of your subject is brought back together at one point on the film. The nearer the subject, the further behind the lens that point falls, so the focus ring screws the whole optical unit forward on its helicoid until the point lands on the film. Between the two halves of the glass, the aperture blades set how wide the bundle of light can be.
Drawing…
The two dark views show the spot that one point of the subject, 1 m away, makes on the film: the lens's point spread function. Focused anywhere else, it is a disc, and closing the aperture shrinks it; that is depth of field. Focused at 1 m and wide open, most of the light still lies in a halo round the point: the outer parts of the glass bring their light together a little in front of the film (spherical aberration, in this textbook design). Closing down cuts the halo away, until from about f/8 the small opening itself spreads the light into rings (diffraction). So the spot is smallest in between, here at f/5.6 to f/8.
Worked out with wave optics, for green light (550 nm). Each step of the aperture lets in half as much light.
2
The rangefinder shows when it is right
You do not look through the lens. You look through the big window beside it, and in the middle of the view a small patch shows the scene a second time, seen from the small window a few centimetres along. The focusing cam at the back of the lens moves with the glass; a roller rests on it and, through two levers, turns the mirror behind the small window. When the lens is focused on your subject, the two lines of sight meet on it, and in the patch the two images become one.
Press the release, and two curtains just in front of the film run across it, one after the other. The first opens the frame; the second follows and closes it. The light reaches the film through the slit between them: at fast speeds a narrow slit, at slow ones the whole frame, open at once. Then a stroke of the advance lever winds the film on, from its cassette across the gate onto the take-up spool, and cocks the shutter for the next picture.
Drawing…
the curtainslight through the slitlight from the lens
Seen from where the lens is, so the cassette is on the right here.
Every part
Tap a part to read what it does and see it picked out in the camera taken apart.