The way a rangefinder calculates distance?


Been spending a lot of time on here!
Aug 29, 2010
Reaction score
Can others edit my Photos
Photos OK to edit
A rangefinder camera can be looked upon a device that calculates (apprx) distance to the subject - hence why it's called a "rangefinder". The camera measures distance by the means of triangulation - if you know three things about a triangle, you can find any angle and length of the side.


In this (poorly made) illustration, A and B are the viewfinder and the rangefinder windows respectively on a rangefinder camera. b is the baseline of the triangle, the length between the viewfinder and the rangefinder window. I think the effective length is also related to the magnification of the viewfinder. Many are .7x, some 1:1 etc. Alpha represents the angle BAS, beta the angle ABS and S is simply the subject. d is the distance from film plane to subject, if I'm not much mistaken. The formula for finding d:


So, we know the baseline, but how does the camera figure out the angles? As you understand, these angles will vary depending on the distance d: shorter distances means smaller angles, longer distances means bigger angles. So, how can the camera figure out those angles?

I was thinking that maybe the angle of the prism that moves the superimposed image from the rangefinder window has something to say, but I can't really figure it out.

Another thing I thought was this: as we turn the focusing ring, the prism turns (at least on coupled rangefinders). The angle from rangefinder window to subject does logically not change, but the angle of the prism does. What I'm thinking is that maybe as we start at closest focusing range and move toward infinite distance, we go through all angles in-between. When the two images overlap, we must have stopped at the correct correct angle. Basically what I'm trying to say, is this:


But I have no idea, really, and was wondering if anyone here knows how the camera pulls the necessary information to calculate the distance to the subject. :)
It's all done mechanically. First alpha is a right angle or close to it, and stays constant. The system of levers that moves the prism or mirror at B (or on some rangefinders, an intermediate prism) is linked to the focusing movement on the lens. Usually it will be set to correspond with the focus throw of a particular focal length and other focal lengths will be geared to that (ie the rear rangefinder ring on the lens will move as if the lens was the focal length the rangefinder is designed for). For a Leica I'm fairly certain that it is geared to the focus throw of a 50 mm lens. Other cameras, like large format rangefinders, use a cam system, with the cam being designed for the particular focal length it is associated with.

Some cameras use an electronic rangefinder that works by timing the return of a sound, light or IR pulse.
Right, obviously, the angle BAS is pretty much 90*. We look through that! *facepalm*

Right, so we have the baseline and one angle, and the other one is "found" by "experimenting" Experimenting as in actually turning the focus ring.

If we destroyed all the the glass inside the lens, we would still have e workable rangefinder. When the two images overlap, we would have found the distance to the thing we looked at. How does the moving of the glass inside the lens correspond to the found distance? Is that just done "manually" (experimentation) during manufacture of a prototype, for example, and then just "mimicked" for the rest of the production line?

I'm not sure if my question came through, there.....

It's done by calculation during design. They choose a particular focal length (as I said, I'm fairly sure that it is 50 mm for Leicas) and design the rangefinder coupling lever system on the camera to follow the movement of that particular focal length as it is focused. Other focal length lenses need to have the movement of their rangefinder coupling flange geared to imitate the movement of the coupling flange of the chosen focal length. It's all calculated during design.
Okay, I see. That makes sense. The rangefinder is a wonderful device!
So, for interchangable lens rangefinder cameras, is there a major difference here? When we adjust the brightframes dial thingy, do we change some other mechanical aspects of the rangefinder as well?
Yes, you move a mask that covers the unwanted bright frames so they aren't reflected into the viewfinder. Focusing the lens may also move the bright frame, or part of it, to compensate for parallax as best as possible.
Thanks a lot for your help!
Not sure if this was part of your question, but all linked rangefinders I have seen have an arm that is linked to the rotating mirror (B in your diagram). In an old Leica screwmount and many other 35mm RFs, that arm terminates inside the lens mount. As you focus the lens, a ring extends out from the back of the lens. A roller on the arm bears agains the ring and the motion is transmitted to the mirror.
On a Graflex press camera with a side mount rangefinder the arm is connected to the focus rails. The RF has an adjustment screw that allows you to zero the moving mirror for various focal lengths. The newer Graflexes with top mount RFs worked the same way except have a cam in the linkage; swap a lens, install the correct cam and your RF is now calibrated to the new focal length.
Not really on topic but you may be interested to see the rangefinder mechanism in the Leica M8. The following is a link to a fellow who literally took his M8 apart piece by piece and photographed them. In the various photos, you can see bits of the rangefinder mechanism. In the first page, there is a photo of the arm and bearing mechanism that links the lens to the rangefinder itself.

Anatomy of the Leica M8 - Leica User Forum

Unfortunately, the photos are spread throughout the long thread... I'm not sure if there is a album online.

PS> I wouldn't recommend to do this yourself on any camera. Apparently the guy has the skill, workspace, and tools to perform such an undertaking. Yes. He managed to reassemble it to full working condition.
Damn. Can't see the images without registering. Just for fun, a few Epay auctions just for illustration purposes:

Leica III The shiny thing inside the mount at the top is the roller at the end of the RF arm.

Leica Summarit Check out the rear views. The brass ring extends in and out of the lens. The roller bears against the face of the ring.
Not sure if this was part of your question, but all linked rangefinders I have seen have an arm that is linked to the rotating mirror (B in your diagram). In an old Leica screwmount and many other 35mm RFs, that arm terminates inside the lens mount. As you focus the lens, a ring extends out from the back of the lens. A roller on the arm bears agains the ring and the motion is transmitted to the mirror.

As I mentioned in post #2, some rangefinders use a prism in place of a mirror - usually the better ones like Leica - and the coupling isn't always to the prism or mirror at B - it can be to an intermediate prism between B and A.

That ring at the back of the lens may be just the moving part of the lens - it is if it is a Leica 50 mm lens. Other focal lengths have to gear the movement of the ring to match that of a 50 mm lens in that case.

Okay. Clearly the calculations are made for the distance from the viewfinder to the subject. That means that the "camera assumes" that the distance from film plane to subject is equal (or close to equal)?
The camera doesn't 'assume' anything really. The cam and lever system design results in the lens being in the right place to focus the image when the rangefinder image is coincident. Maybe it is easier to think of the cam system on a large format rangefinder. Here a sloping cam on the focusing rail lifts the rangefinder lever up and down. If the lens is focused on an object, then the cam is designed to be the right height at that point to lift the lever to the exact angle that then makes the rangefinder image coincident on the focused object.
Compaq: did you mean that the camera "assumes" that the distance between the subject and the filmplane and the distance between the subject of the baseline of the RF are equal? They don't need to be equal. Often the baseline (AB) is slightly infront of the filmplane.

Most reactions