
How does the Canon Rangefinder Work?
Flynn Marr 03 November 2024
I was corresponding with my friend and fellow collector Magnus in Wellington, New Zealand, and he asked me a question that caught me off guard. Rather than paraphrase him lets look at his e-mail:
With my engineering background I have always understood in general terms how a rangefinder worked so I have never really paid attention to this subject. And when Magnus posed the question I realized that I did not know in detail how they worked and I did not know how the three-mode optical viewfinder worked. And I had never gone into it on this website. Surprise number one.
Magnus said he could find little or nothing about “how the lens communicates with the rangefinder”. I did my own search and he was right. I too could find nothing that explained it in detail. Surprise number two!
So let’s correct this oversight right now. In what follows I will attempt to answer all of the questions that Magnus raises.

We have a Newsletter
There is a Newsletter for thecanoncollector.com to keep you up to date on what we are posting. Try it!
First some basic geometry. The camera above is taking a picture of a tree. The tree is centered in the viewfinder. If the tree is at infinity, which it is effectively if it is over 200 feet away. The angle ß is zero degrees. Now, as the tree gets closer and closer to the camera that angle gets greater and greater as you can see in this diagram. It is this simple fact that is the basis of the rangefinder on the camera. The size of that angle is related directly to the distance of the subject from the camera. For the more technically minded, the distance to the Subject can be expressed as
L = d / Tanß where d is the distance between the Rangefinder and Viewfinder Windows
But forget the math. Just make sure you grasp the concept before we go on.
The second fact we have to understand is that the lens on the camera focuses by moving closer to or further from the surface of the film. For a subject at infinity the lens will be as close as it gets to the film and at this point the distance to the film plane is the focal length of the lens, approximately. It is a little more complex than that for multi element lenses. As a subject moves towards the camera the lens moves out to keep it in focus. The trick for the rangefinder is to somehow relate the angle ß to the distance the lens is from the film.
Most 39mm LTM (Leica Thread Mount) lenses have an inner brass barrel that moves in and out as the lens is focused. The rear edge of this barrel is polished smooth for a “feeler” to follow as it moves in and out.
The Canon Rangefinder
Getting back to the camera, light from the subject enters the Viewfinder window at a perpendicular angle to the camera because the camera is aimed at it. Some light also enters the Rangefinder window which enters at that angle ß from the perpendicular that we already discussed. It is reflected to the optical assembly behind the Viewfinder window. The paths of the light rays are shown by red lines in the image to the right.
In the box behind the Viewfinder window is a plate of clear glass with a partial reflective surface set at 45 degrees to the light path. Most of the light from the scene passes through it and on to the eyepiece with a portion being reflected away to the side and lost but there is still enough going to the eyepiece for a reasonably bright image. The light from the rangefinder window is reflected by a 90 degree prism onto the same partially reflecting mirror where a portion of it is reflected 90 degrees and passed on to the eyepiece.
If the light from the Rangefinder window arrives parallel to the light entering the Viewfinder window the images will superimpose exactly and appear as a bright circle in the center of the field. This would mean the images are both at infinity or at least over two hundred feet or more away.
As the subject moves closer to the camera that angle ß gets larger which throws the light through the Rangefinder window to the side creating a double image in the eyepiece in that circular area.. However, if we had a way to rotate the prism behind the Rangefinder window we could realign the images. If we could tie that prism rotation to the focal position of the lens we could put it all together in a rangefinder coupled camera. That is our next task.
The Subject is viewed through the Viewfinder window, on the right, which enters at 90 degrees to the camera. But some light enters into the Rangefinder window on the left where it is reflected 90 degrees into the assemblly behind the Rangefinder window where it is combined with the light from the subject, as we shall see.
The light entering the Rangefinder window creates a circular image in the center of the scene. For distant objects it aligns perfectly but as the subject comes nearer the camera it moves to the side by an amount depending on the angle the light enters the Rangefinder window.
What we do is put the prism on a rotating arm with a pivot under the prism. The arm is linked to a “feeler” that rides along the rim of the brass barrel in the lens, shown above, or is pushed by the tab found in longer lenses which is also shown above. If the linkages are designed correctly the images can be made to line up at exactly the correct focus position of the lens.
All LTM lenses have a standard movement designed into the lens so that no matter what LTM lens is used the rangefinder will work correctly.
So, if we put it all together, we get something like the diagram above. Now understand that no camera was ever built this way. This is much too crude but it explains how it works. The Prism A provides a 100% internal reflection of the small circular image from the lens in the Rangefinder window to the left. This diagram shows different light rays with the prism in the same position meaning the rangefinder image will be misaligned.
The light entering the Viewfinder window passes through a 45 degree partially silvered mirror. Some of the light is reflected to the left and lost but enough passes through to create an image in the eyepiece. The light from the Prism A comes in from the right and some passes through the Mirror C but enough is reflected to the Eyepiece to create a small circular (it could be any shape depending of the geometry of the Rangefinder window and optics behind it) image superimposed on the field of view.
The Mirror C is a partially silvered mirror. It is not a pellicle mirror as Magnus suggests. A pellicle mirror, such as first used by Canon in the Pellix camera, may be partially silvered but it is thin, usually made of a plastic material as in the Pellix, because to make it of glass would be too delicate.
If all of the linkages are designed correctly, when you move the lens into focus the “feeler” B resting on the rear
of the lens will move with it, held against the edge of the lens barrel by the Spring D. Linked to the Prism Arm the prism moves to align the images in the viewfinder. The scene stays stationary and only the small circle moves across it.
With careful design, all LTM lenses move in the same manner with identical spacing, so that any LTM lens will function on the camera. Leitz developed this system for their Leica camera but now any lens with a 39mm LTM mount will fit on the Canon rangefinder camera be it from Leica, Canon or dozens of other lenses created with this mount. Very clever!
So there you have the rangefinder. What we have described is what you will find on a Canon SII. Canon was innovative with that camera because they combined the Viewfinder and the Rangefinder in the same image unlike the Leica II and III which separated them and had two eyepieces, one for the Rangefinder and one for the Viewfinder. In the early cameras the view was small and the edges were fuzzy but with each new camera the image got better and brighter until we get to the brilliant images in the last of Canon’s rangefinders, the Model 7 and 7s. But the combined Viewfinder/Rangefinder in all of them worked the same way.
Canon was not happy with the Viewfinder in the SII. The main concern was that with longer lenses focus became very critical as the Angle ß was so small that it was not possible to line up the superimposed images accurately. To overcome this they created their 3 Mode Viewfinder which was first introduced with the Model IIB.
The device is actually quite simple. Everything is if you know how to do it! They introduced a small block which sat in the light beam going to the Eyepiece. A lever they called the “Range Finder Magnification Changing Lever” was added to the left end of the camera under the rewind knob. We’ll call it the Viewfinder Lever. It had three settings, F, x1.0 and x1.5.
As the Lever was moved the small block rotated so that the light to the Eyepiece passed one way through the lenses in the block, or it passed through unaffected at all, or it passed the other way through the lenses.
In the image on the right you can see the small block I am talking about. In this image it is positioned so that the light can pass through the cube without being effected. It is simply an open path. But you can see the lens on the side. There is a lens on both sides actually and if you look one way through them the image is magnified but if you look the other way it is reduced, much like looking through a telescope one way and then the other.
The diagram below shows the three positions that the
cube can be in. The F setting reduces the view and
approximates a 50mm lens.
The x1 setting is the open path through the
cube. The image is as it comes from the Viewfinder and approximates a 100mm lens. And finally at the x1.5 setting the light passes through the lenses the other way and the image is magnified. In this setting the scene approximates a 135mm lens.
With the magnification the ovelapping images are also magnified and they can be aligned more accurately which is important for longer lenses which are more critical in focusing further out from the camera.
Problems with the Viewfinder
The first problem is that the viewfinders, especially the early ones, were dim. They worked well in bright light but one struggled to get good focus in poor light. The later viewfinders did not suffer so much from this because the window became larger admitting more light. By the time we get to the Model 7 the Viewfinders are bright and much easier to use.
However, the biggest problem was that Angle ß. When one was using long lenses focusing was usually at a point a long way from the camera. For instance, if you were on the sidelines of a football game you wanted a long lens to capture the action that was often 50 or 60 feet away. That means that the angle was getting vanishingly small. And longer lenses are more critical to focus and have a shallower depth of field. These factors meant that in practice in a small 35mm camera the maximum practical focal length was around 135mm. That is why rangefinder cameras are seldom seen with longer lenses and this was a real limitation.
The answer to both problems was the single lens reflex camera with through the lens viewing. But that is a conversation for another time.
Occasionally “gremlins” will put your rangefinder out of whack. The images may be misaligned vertically which is annoying but not fatal to good focus. However, they can be misaligned horizontally which means that the lens’ focus is wrong. That is more serious. But the Rangefinder is easily adjusted.
For horizontal adjustment you remove the bezel around the Rangefinder window. Underneath you will find that there is a ring around the lens with two spanner notches on opposing sides. Set the lens to infinity and then look at a vertical object and note the degree of misalignment. Insert the appropriate spanner and rotate by a quarter turn in either direction. Check the viewfinder to see the result. If you went the wrong way just go the other way. Then keep adjusting and checking until the item at infinity is aligned horizontally.
To adjust vertically, take the screw in the front of the camera next to the Viewfinder window out. It holds nothing. It is just a cover to keep out dirt. Inside there is a slotted screw head. Adjust it a bit and check the effect. Keep adjusting and checking until a horizontal object is perfectly aligned. Then put the screw and bezel back and snug them up so they don’t fall off later.
Generally these adjustments are not necessary. The two main reasons you may need to do this is if the camera has had a bad nock or someone got their fingers where they did not belong. Otherwise the camera rangefinders are pretty robust.
Behind the bezel on the Rangefinder window is a notched ring that can be turned to adjust the image horizontally. Set the lens to infinity and then align the images and done! Put bezel back on!
And in Conclusion ….
And that is a brief explanation of how the three mode rangefinder camera works. I hope I have explained it well. If you are intending to acquire a Canon rangefinder camera as a shooter there are two things I would say. Research your prices on E-bay and then buy a camera in excellent operating condition. So long as it is operating correctly a few scuffs and bangs don’t really matter. Buy a late model camera such as a Model VT, VI-L,P, or Model 7. You get superior viewfinders and more robust construction.
On the other hand, if you are buying for a collection, appearance is very important but you will pay a premium for it. Always buy the best camer you can afford because they will hold their value longer.
This website is the work of R. Flynn Marr who is solely responsible for its contents which are subject to his claim of copyright. User Manuals, Brochures and Advertising Materials of Canon and other manufacturers available on this site are subject to the copyright claims and are the property of Canon and other manufacturers and they are offered here for personal use only.