The Canon Rangefinder
Canon Model 7

The viewfinders
of the Canon Model 7, 7s and 7sZ are essentially
the same in construction and operation and they are the finest  viewfinders for brightness and contrast to be found on Canon’s rangefinder cameras.

Canon VI-T Viewfinder

The frame lines visible in the VI-T viewfinder are deposited on a lens surface just inside the eyepiece of the viewfinder. These are only visible to the eye when the magnification when the magnification is set to 50mm.

Model 7
Framelines

Flynn Marr and                               12 November 2024
Todd Belcher

Canon introduced its three magnification viewfinder with the Model IIB in 1949 and it remained their standard viewfinder until the introduction of the VT delux and VL2 in 1958. There were improvements in the optics along the way but the basic system did not change.

Competition in the camera industry late 1950’s and early 1960’s in was intense and a feature of this included a drive for brighter and clearer viewfinders. To keep up Canon had to look for ways to improve their own viewfinders and so it was that the introduction of the VI-L and VI-T in 1958 brought improvements.

These were the last two Canon cameras to use the three magnification system that had been pioneered by them in the IIB. The rotating block, which had turned on a vertical axis was tipped over and now rotated on a horizontal axis. Frame lines for 50mm and 100mm lenses were added when the viewfinder was set to the 50mm lens setting.

All of this becomes obvious when you look into the eyepiece of one of these cameras with a small light. The frames by are visible applied to a glass element just inside the eyepiece. They are so close to the eyepiece exit that the eye does not see them directly. However, on the 50mm setting the rotating lens assembly presents a partially silvered concave lens surface which reflects these frame lines so that the eye can focus on them much in the manner of an Albabda style system. The lines appear to hover over the image in the viewfinder and be independent of eye position.

These frame lines were corrected for parallax by their glass element moving diagonally resulting in the  frame lines appearing to move downward and to the right as the lens was focused closer.

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The next camera from Canon was the very popular Model P which did away with the three magnification viewfinder and went all in with frame lines for 35, 50 and 100mm focal length lenses. Again the frame lines can be seen deposited onto a glass element near the eyepiece exit and the partially silvered concave mirror is visible behind them. These too were parallax corrected but on the Model P the parallax pin in the accessory shoe was also eliminated never to appear again.

I think this arrangement, which gave an excellent image, had two problems. First was the complexity of the frame lines being always present in the viewfinder image. It was distracting. But possibly more serious, the partially silvered mirror had to reflect a goodly portion of the light reflected off the frame lines which meant a dimmer image overall.  Now, these are my own conjectures but they seem quite logical to me.

The Rangefinder

You will notice that I have not mentioned the rangefinder in these cameras. That is because we covered that subject in my earlier post entitled “How does the Canon Rangefinder Work?”. All of Canon’s rangefinding cameras worked on the same principals. There may have been improvements in the optics but the basic structure and theory did not change. It is the viewfinders that evolved.

The Model 7 Viewfinder

Canon Model 7

Framelines for 35mm, 50mm, 85 and 100mm and 135mm are selected by turning a dial on the top deck. This control did not affect magnification which stayed at 1:1 throughout.

Canon Model 7

A new window was added to the front of the top deck on the Model 7 and 7s filled with a translucent plastic screen to illuminate the new frame lines.

The Model 7, 7s and 7sZ were the last of Canon’s rangefinder cameras and they had the most sophisticated of all of the viewfinders. They kept the fixed 1:1 magnification of the Canon P and they retained the frame lines but created a system to dial in only the frame lines the user wanted be it 35mm, 50mm, 85 or 100mm or 135mm. The user did not have to deal with frame lines they did not require. By adding a new window on the front of the top deck to illuminate the frame lines they did not deduct light from the viewfinder which meant more light for the image overall.

The mechanism to do this was complex. Not only did the camera change the frame lines visible in the viewfinder but it moved them diagonally to compensate for the inevitable parallax present in viewfinder cameras. To understand this mechanism is going to take a bit of effort.

Canon Model 7 Rangefinder

I said this would take a bit of effort! Lets go over this diagram. The rangefinder and how it connects to the lens is not completely shown. Only the optical parts for the viewfinder are here. Light from the Rangefinder Window reflects to the left of a small 1st surface mirror into the Partially Reflecting Rangefinder Mirror.

All Canon Rangefinder cameras use a prism to reflect the light by 90 degrees inside the Rangefinder Window. All that is, except for the Model 7 and 7s which both use a 1st Surface Mirror. Every time light passes from one medium to another of different refractive index, such as air to glass or glass to air, a small percentage of the light is reflected back. This means that the prism does dim the image by a few percent.

If you replace the prism with a mirror you have a different set of problems. A glass plate silvered on the back side, as most mirrors are, does reflect a small portion of the light hitting it from the front surface towards the eyepiece. This can cause a secondary image to muddy the clarity of the rangefinders split image. This does not happen with a prism. To avoid this you can silver the front surface, or first surface, of the mirror. This eliminates the problem and gives a slightly brighter image. But, the silvered surface is now not protected from the air and moisture and it is incredibly delicate. Cleaning it is a very delicate procedure, even if a transparent coating is applied over it. Such a coating introduces it own problems. 

However, in spite of all of this, in the Model 7 and 7s Canon chose a 1st Surface Mirror which is visible in the image below on the right.

Canon Model IIB

This is the rangefinder in the IIB and just left of center in the image is the rangefinder prism that reflects the rangefinder image by 90 degrees into the rangefinder partial mirror prism.

Canon Model 7

This is the rangefinder in a Canon Model 7. It is the same as in the 7s. Notice that the Rangefinder Prism has been replaced by a 1st Surface Mirror. All Canon rangefinder cameras use a prism except for the  Model 7 and 7s. (Image by Todd Belcher)

In the Image on the right, Image 1, of a Model 7 with the top deck removed the Rangefinder Reflecting Mirror and the Framelines Selector Assembly are removed. They would have been at positions B and A respectively (light paths for the frame lines are shown in green).

Light enters through the Frame Lines Diffusion Screen in the Frame Lines Window and evenly illuminates the Framelines Selector Assembly. From there the light for the selected frame lines reflects internally in the Framelines Reflecting Prism. This is a large prism because it must pass the largest of the frame lines, 35mm, at full size.

The frame lines then enter the Partially Reflecting Framelines Mirror. This is glass block that was sawn in half at a 45 degree angle and partially silvered before being glued back together. However, in this case, there is an unslivered portion in the middle for the light from the Rangefinder Reflecting Mirror to pass through unaffected.

In this way the selected frame lines appear superimposed over the image in the viewfinder and the double image for focusing appears in the middle unaffected.

In Image 2 on the right the Rangefinder Reflecting Mirror and the Framelines Selector Assembly (right of center) are in place. They are normally buried under the light meter and other linkages which have been removed so we can see what is happening.

In  Image 3 the light meter has been installed. The Framelines Selector Cam rotates on the screw through it. The hole in the Cam accepts a pin on the bottom of the Framelines Selector Dial on the top of the camera. As the Cam is turned it causes the Framelines Selector Arm to follow its contour which causes the Framelines Selector Pin to move to the left or right. This pin rests in a slot on the Framelines Selector Assembly and as it travels it moves the Framelines Mask in the Selector Assembly.

As we shall see below this action selects the frame lines that will be displayed in the viewfinder. And there you have it. You can see how the dial on the top of the camera is connected to the Framelines Selection Assembly. Now all we have to do is see how that mechanism works.

All my life I have enjoyed taking things apart to see how they worked. It was a bit of a passion actually. I could take anything apart. The problem was that I was not so good at putting things back together. I always made the classic mistake of thinking I would remember where everything went. Well, life does not work that way! I have to tell you that several cameras and lenses have paid the ultimate price in my quest for understanding. I thank them for their service.

My friend Todd Belcher fortunately has the gift of reassembly. He actually is very good at it and repairs these cameras, something I do not do. All of the pictures of the mechanisms inside the Model 7 shown here are his and he has educated me on how all of these pieces work.

 

Canon Model 7

Image 1   In this image of the viewfinder on a Model 7 the top has been removed and much of the mechanism that normally covers the components has been taken out. The Rangefinder Reflecting Prism wouold normally be at position B and the Framelines Selector Assembly would normally be at position B but they are both missing in this photography. (Image by Todd Belcher)

Canon Model 7

Image 2   The Framelines Selector Assembly is visible just to the right of center. The frame line desired by the user is chosen by the Framelines Selector Pin moving horizontally  to the left or right as is discussed below. (Image by Todd Belcher)

Canon Model 7

Image 3   Here the viewfinder is assembled as it appears when the top deck of the Model 7 is removed. The selector dial on the top deck has a pin on the bottom that fits into the hole on the Framelines Selector Cam and turning this Cam moves the Framelines Selector Arm back and forth. The Framelines Selector Pin engages with the mask in the Selector Assembly and moves it left or right to select the desired frame lines. (Image by Todd Belcher)

The Framelines Selector Assembly

The frame lines magic happens in the Framelines Selector Assembly. I have no idea what Canon calls these parts. The names here I have created so that we can refer consistently to them.

The Framelines Selector Assembly is composed of three parts. The first is the Frames Holder, shown below on the left in Image 5, which has two feet on it that are bolted to the frame of the camera. Its only purpose is to hold the other two parts securely in the camera but allowing them to move as required.

The middle part is the Frames Template, shown below in the center in Image 6. As you can see, all of the possible frame lines are present and they allow light to pass through the template for whichever frames the user has selected.

Canon Model 7

Image 4   The Framelines Selector Assembly is shown here assembled and ready to be mounted in the camera. It is the heart of the viewfinder in the Model 7 cameras and it is essential to understand its operation. It is a three piece device and we shall look at each piece in turn to decipher their functioning.

Canon Model 7

Image 5   The Frames Holder is a structure that holds the other two parts and allows them to move relative to each other. It has two feet which bolt securly onto the camera frame.

Canon Model 7

Image 6   The Frames Template mounts on the Frames Holder by slipping the Parallax Guide Pin and Alignment Pin into the Parallax Guide Slot and Alignment Pin Slot.

Canon Model 7

Image 7   The Frames Mask mounts on the Frames Template by slipping the two Pins on the back of the Template into the Framelines Guide Slots on the Mask. The mask can move horizontally relative to the Template.

The third piece is the Frames Mask, shown in Image 7, which masks the light passing through the Frames Template allowing light to pass only for the selected frame.

Parallax Correction

All of the frame lines are corrected for parallax. That means that as the camera is focused closer the frame lines move down and to the right in the viewfinder. This is accomplished by moving the Frames Template, and the Frames Mask which is attached to it, diagonally in the Frames Holder. 

If you look closely at Image 2 you will see a vertical pin labeled “Parallax Link Pin”. This pin moves as the lens focus changes and as it moves it pushes on the Parallax Control Pin which is attached to the Frames Template. These pins move horizontally but the Frames Template is prevented from doing that by the Parallax Guide Pin and Alignment Pin riding in the Parallax Guide Slot and Alignment Pin Slot in the Frames Holder. So the frames move diagonally as it is the only direction open to them. In this way the camera viewfinder compensates for parallax regardless of which frame lines the user has chosen..

Frames Selection

Are you still with me? I said this would be complex! Now have another look at Image 3. As the user turns the selector dial on the top of the camera the Framelines Selector Cam turns which moves the Framelines Selector Arm which in turn moves the Framelines Selector Pin which is attached to it. In image 2 you can see the Framelines Selector Pin resting in the Framelines Selector Slot in the Frames Mask. The mask moves as the Selector Pin moves which selects the frame lines to be shown. The Frame Lines Selector Slot is a long diagonal opening. This allows the Mask to keep its position as the assembly moves to adjust for parallax.

Now lets look at how the Mask works. The Mask is mounted on the pins on the back of the Frames Template and it can only move horizontally as it is constrained by the horizontal Framelines Guide Slots. As it moves it covers and uncovers the various frames. 

Let’s look at the Frames Template from the back so that we see it as it is viewed in the Viewfinder. You see the two pins that we slip the Frames Mask over and how the slots allow the Mask to move horizontally. As already mentioned, the diagonal slot in the tab  allows the Mask to be positioned horizontally and at the same time allowing the Mask and the Template to be adjusted for parallax by moving diagonally.

Canon Model 7

The Frames Template is the middle layer in the Framelines Selector Assembly and it is generally opaque with the various available frames and lettering etched transparently into it.

Canon Model 7

The Frames Mask has clear openings  in it that allow light to pass through. The Mask mounts on the two pins on the Frames Template and can move horizontally across it. As it does the various frames are shown one at a time.

To see how this works have a look at the following four diagrams. As the Mask slides across the Template the frames for 35mm, 50mm, 85-100mm and 135mm are revealed one at a time and light from the Framelines Window Diffusion Screen is allowed to pass through.

Canon Model 7

35mm Frame Lines

Canon Model 7

50mm Frame Lines

Canon Model 7

85-100mm Frame Lines

Canon Model 7

135mm Frame Lines

And that is how the Model 7 Viewfinder, its parallax correction and selectable frame lines works. It is a complex mechanism of the type that leaves me in awe. How did someone figure this all out.

Now, a word of caution: this is a delicate mechanism and not easy to calibrate. There are several surfaces that will attract dust and dirt and cleaning them is a major chore. Don’t tinker with it! In fact, don’t take the top deck off your camera. There is a substantial chance you will ruin it or at least throw off the delicate calibration. Be satisfied that you know how it works. If your camera is malfunctioning find an experienced repairman. And you can thank me later for this piece of advice. I just saved your camera for you!

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