This is a restored copy of the document by courtesy of Peter R. from rangefinder.ru
After reading this video makes more sense.
After reading this video makes more sense.
Zeiss Ikon Rangefinder Adjustment
This article is a rewrite of three posts I made in the Rangefinder Forum (RFF) regarding the dismantling of the Zeiss Ikon rangfinder camera (ZM) and the adjustment of it's rangefinder mechanism. The original posts were in response to various dicussions about the subject and particularly about the adjustment screws that can be found under the camera accessory shoe.
It goes without saying that anyone using this information does so at their own risk. I make no guarantees the information is accurate or complete.
I have skipped over the practicalities of getting access to the various points and have, instead, included this at the end.
So what are these screws under the accessory shoe anyway?
I first need to clarify that I have not adjusted these screws under the accessory shoe so my understanding of what they do is supposition based on my observations - not on actual experience.
To understand the purpose of the these screws, an understanding of the whole rangefinder mechanism may help. I've done a couple of diagrams that I hope will help. First the general optical path.
On the far right is a prism which rotates when the focus is adjusted. Next to it is a glass block then some surface mirrors then finally the prisms that combine the images in the viewfinder. This is pretty standard stuff but the main point is that we have a prism that has to be rotated a little as the lens is focussed.
To achieve the linkage between the lens cam follower and the prism there is a series of levers thus...
The lens cam follower moves in and out and causes lever A to swing around point Q. Lever A engages the cam end of lever B and causes lever B to move back and forth also. Lever B is rigidly fixed to lever C which is hinged at P where the prism is mounted. So the forward and backward movement of the cam follower causes the prism at P to rotate. The amount of rotation is controlled by the angle of the cam at the end of lever B and this can be adjusted by the rather complex looking coupling between levers B and C.
Now look more closely at the join between levers B and C.
This is what you can see if you look below the accessory shoe. There are three screws that I've labeled X, Y and Z. X is the black, cross head, locking screw (circled in the photograph) and Y and Z are slotted adjustment screws. In the centre we also have two fixed studs which are located in the curved slot in lever B. The two adjustment screws Y and Z are eccentric screws - that is, their heads are offset. If we unlock the locking screw X and adjust Y, lever B slides back and forth as the eccentric head of Y pushes it. The movement is controlled by the two fixed studs so lever B can only move in a slight arc.
If you look back at the previous image, you will see that moving lever B in this arc will alter the pitch of the edge along which lever A operates. The overall effect of this is that adjusting screw Y will change the amount that the lever B and C assembly moves for a given amount of movement of lever A. (with me so far???)
So what of screw Z? Well this isn't an adjustment screw as such. What it does is to keep lever B in close contact with the two fixed studs.
The following picture shows this join of levers B and C in the camera. It's not possible to see the whole arrangement without removing the rangefinder assembly above it but I strongly recommend you don't do this as it will upset the rangefinder calibration - and there's actually no need to do it anyway.
You might also note in the above picture the sealant on the locking screw. I think it's also worth noting that any attempt to adjust screw Y without first unlocking the locking screw will probably result in damage (I guess there's some people who didn't want to hear that). Similarly, if screw Z is adjusted you will either introduce slop into the system or you will cause damage by pushing lever B up against the fixed studs.
My conclusion to the above, and this is based also on what follows, is that screw Y is to adjust the close focus of the rangefinder.
So if that adjusts the close focus, what adjusts infinity focus?
The prism previously mentioned which rotates while focusing can be rotated manually to adjust infinity focus. The prism and it's adjustment screw can be found under the shutter speed dial assembly. The adjustment screw is circled green in the following picture (for now, ignore the one circled in red - we'll get to that later). The prism itself is held in the metal circular piece with the blue dot on it in the picture and can just be seen.
Here's another picture of the arrangement with the camera cover off so it can be seen more clearly.
If you look either side of the crosshead screw you can see two slots which is the slot of the actual adjustment screw. The crosshead is a locking screw and fits down the centre of the adjustment screw. The adjustment screw is eccentric and if the locking screw is loosened, and the adjustment screw turned, the prism will rotate slightly in it's mount so adjusting the alignment of the image in the viewfinder.
The locking screw will probably be sealed (mine isn't because I removed the sealant) and I suggest you start by removing all the sealant before trying to undo the locking screw. This will allow the locking screw to turn without the risk of upsetting the present setting of the adjustment screw underneath. Also, just slacken the locking screw the absolute minimum as the screw also holds the prism assembly together and if you take the screw right out, it all comes apart.
So, in summary, what we have are two adjustments of the horizontal alignment. My conclusion is that the first one, that can be found under the accessory shoe, is, as I said, the close focus adjustment while the one next to the prism is the infinity adjustment. I have spent some time pondering this as, at first, I wasn't sure which was which but I finally decided it was this way around when I thought of them as being a Gain and Zero control (something more familiar to the electronic guys amonst you of which I'm one). The adjustment under the accessory shoe effects the amount of movement of the prism for a certain amount of movement of the lens cam follower. I interpret this as a gain adjustment. The movement of the prism will have equal effect over the full range of movement so this is more like a zero of offset control. The question then is, at what point of the range do you adjust the zero control. In this case, I'm guessing that the arc through which lever B moves when screw Y is adjusted is centred on or about the position where lever A will contact lever B when the lens is on infinity. This is the only fixed or known point so it would make sense. If this is so, then adjustment of screw Y when the lens is set to infinity would have no effect so this must be the control for close focus and adjusting the prism is for infinity adjustment. I suspect, in reality, adjustment of either is likely to upset the other so checking both close focus and infinity focus would be a good idea after any adjustment is made. In my own case, I adjusted the prism for infinity focus but I found no problem with the close focus afterwards.
So how about vertical adjustment?
Look at this picture of the way the prism (circled green) is mounted. Note especially the two strips of metal below it circled red.
The lower of these two strips is the end of lever C. The prism is actually mounted on the upper strip of metal and, between the two there is a small adjustment which moves the upper strip up and down. In the next picture you can see this adjustment has a pair of flats on it so it can be rotated and it also has sealant on it to lock it in position. This is obviously an adjustment and it alters the vertical alignment of the rangefinder image by very slightly tilting the prism.
Note that this is not accessible without removing the top cover but there is a possible alternative.
In the following picture you can see another eccentric screw sitting in a slot. This is the same screw that can be seen through the speed dial aperture in the top plate shown in a previous picture. The screw adjusts the tilt of a glass block (maybe some form of prism) which is attached to the pivot point just to the left of the adjustment screw and this would effect the vertical alignment of the image. The problem is that this assembly is sealed very well and is obviously not intended to be adjusted. I suspect it's a coarse setting which, once set in the factory, is not intended to be moved again.
However, I did try turning the screw and, even with everything sealed up, I did get some vertical movement of the rangefinder image. Because this screw is accessible without removing the top plate, though difficult to get at, it may be a possible solution to vertical alignment problems. But an altogether, possibly, easier way to adjust the vertical alignment is to bend (yep I said bend) the end of lever C up or down. This end of lever C with the prism mounted on it is not supported at all by the body of the camera, it's only attached to the rangefinder assembly, so it will flex up and down a little. I found just by putting a little pressure on it either up or down, I could make small adjustments to the vertical alignment. It should be possible, I think, to do this via the shutter speed selector aperture in the top plate so, as long as any error is small, this may be a quick and dirty solution.
OK, I'm up for it - how do I get it apart?
The only thing you really have to do is to remove the shutter speed dial so we'll deal with that first. Remove the pin faced screw in the middle of the dial while holding everything down as there is a large spring underneath and, if you don't hold the dial in place, parts will go into orbit. With the screw removed, carefully lift out the parts and note their position.
Reassembly is a bit more difficult so I'll describe that in more detail.
The parts you should have prior to reassmbly are shown here in the order they are fitted.
They are (from the right) the fixing screw, a washer, the main dial, a washer, the film speed dial, a spring, a plastic housing and a metal plate.
This shows how the plastic housing and the metal plate mate together. You might find it easier to do this first before putting them on the spindle or you might find it easier to do them one at a time. Note the forked piece which mates with a lever in the camera.
Here's the lever in the camera. The lever rotates smoothly (no click stops) by about 180 degrees so, if the fork has engaged it properly then you'll be able to rotate the plastic housing by the same amount. If it rotates more than that, you've missed.
Once you have the plate and the housing in place, you can sit the spring in position.
Then you need to fit the ISO plate. Note there is a lug on the plate which engages with the notch in the plastic housing.
When you fit the ISO plate you are compressing the spring so this is where it gets interesting as you need to hold the ISO plate down against the pressure of the spring while fitting the remaining parts.
Fit the washer next and make sure also the notches in the spindle and the metal plate, fitted first, now line up. Then fit the main dial making sure the lugs fit in the notches correctly. Note there's two ways the main dial can fit. If you can't see the ISO speeds through the notch in the main dial, it's the wrong way around.
And finally fit the screw and it's washer.
If you need to do anymore than just remove the speed dial, the next thing you are likely to want to do is remove the accessory show to access the screws under it.
NOTE: If you decide to remove the top plate completely, it's not necessary to remove the accessory shoe.
To remove the shoe, first remove the cover plate by lifting the edge of it with a small screwdriver and pulling it out towards the rear of the camera.
Next remove the four screws and lift the shoe away. Also remove the contact piece underneath which will now be loose.
Refitting is just the reverse.
If you decide to remove the top plate completely, start by removing the speed dial as previously described. Then remove the wind-on lever by first unscrewing the screw in the centre of the lever. There is no method of gripping this screw so use a peice of rubber or a latex glove to provide enough grip to unscrew it.
Lift off the parts of the lever noting the order they are fitted. You should have the following...
Nothing else needs to be removed to lift the top plate. Remove the two screws at the back edge of the top plate and also the two screws just inder the edge of the front leathers (one either side).
Lift off the top plate lifting the wind-on end first.
The top plate will still be attached by the wires to the flash socket. It is possible to remove the socket from the top plate but it's easier to desolder the wires.
You should now see something like this.
Refitting is the reverse but an important point is to make sure the on/off switch on the top plate is in the OFF position before refiting the cover. The actual switch remains on the camera and the part that remains with the top plate is just a lever. If the lever is in the wrong position when refitting the top, it will likely push apart the switch (yep, I did it). Here's the switch.
This isn't a major disaster as long as the switch isn't physically broken but the hinged part of the switch may get disengaged. It can be put back together, assuming you can find the part, but it's very small and not easy.
That about covers it. If anyone does any of this and wants to add something to the above, please get in touch.
Copyright © 2008 Peter Robinson. All rights reserved.