Rev.1

The foundation to good image clarity is a precise Lens Flange to Film Plane distance or Camera Working Distance. All of our further tests for lens and rangefinder precision will be based upon knowing that this critical Camera Working Distance is set to perfection.

To verify the camera working distance we will need a depth gauge or a slightly modified Vernier Caliper. I added this base to help stabilize the vernier and to make a larger working envelope for the depth measurements. Actually if I had to make this part again I'd make it bigger.

There may be some uncertainty with a Contax/Kiev lens mount just what exactly is referred to as the Lens Flange. The front surface of the outer mounting flange circled in blue is the reference for the lens flange. We'll make measurements at the four corners of the negative window indicated by the yellow X.

All measurements will be done with film in the camera and not to the pressure plate. The pressure plate does not ride at the same distance as the film plane. There is a little gap between the pressure plate surface and the film plane as the pressure plate rides on the two outermost rails. You will be sacrificing a roll of film for this measurement but it's good to have a test roll of film on hand to check things out with from time to time.

Start a measurement by referencing one of the outer mounting flanges and zeroing out to that surface.

Move your depth gauge to measure at or near to the reference flange you set zero at. Re-measure a couple times to make sure you didn't misalign something when you took your reading. Just a note regarding the top right corner, the inner lens mount latch will make it difficult to directly make this measurement. I have some inclination to be doubtful about accuracy of the top right corner reading. I am leaning towards using an alternate method to measure working distance. As well, measuring by this method entrusts that the very front surface is exactly flat and true.

Make a little diagram and record your values. The camera working distance value should be 35mm +0.01/-0.03 as per Peter Tooke's book, Zeiss Contax Repair Manual. How does it look? Within tolerance? From my example here I have one corner that is a little out of spec. If your method is suspect there is another way to gain these measurements. 

If you have a boned Jupiter-12 then you can use the lens mount collar as an aid if you find it difficult to hit the rather small outer lens flange.

This method will require you measure this distance accurately. You might need the assistance of a machine shop to help you measure this. I measured at many intervals using a Mitutoyo Granite Block surface and a Digital Height Gauge. I am very certain of these measurement values.

You may even need to true up the mating surface to be nice and exact. If you can hold your tolerance to +/-.01mm then all the better because if you remember, our overall flange to plane tolerance is only +0.01/-0.03mm.

Once you've discerned the precise thickness of the lens mount collar you can make your depth measurement to the film plane easier. How does this compare to the initial method?

From the results I logged, it would appear that the two methods are not in agreement with each other. I have even constructed a bigger sturdier base and get similar values for the Alternate Method. I am tending to agree with the Alternate Method as a better way to gain measurements as it affords an unobstructed surface all around the lens opening to reference.

I think no matter which method is adopted repeatable results are necessary for any further tests and adjustments especially when it comes to shimming the lenses to match the camera working distance.

Adjustments

Now that you know how much the lens mount is out from the specified 35.00mm +.01/-.03, all that is required is to shim to correct.

You can use any material to shim with as long as you have a broad selection of the right thickness available. I have used negative film base, paper, even aluminum cooking foil for very slight shimming changes. One good source for shims is the hobby shop. I have some etched brass frets of washers made by Athabasca Scale Models that are .008" (.20mm) and .004" (.10mm) thick.

Once you have determined the amount of shim you need, just slip it under the mount base.

Tighten all the screws and re-measure all four corners again. Even if you change just one corner's shim you should measure all four corners.

An Interesting Discussion

I had an interesting discussion recently regarding the correct Lens Flange to Film Plane distance. The following are excerpts from the thread:

I think the distance you want is 34.85 mm. There is a
table of flange-to-film distances on this website:

"http://www.kineoptics.com/index.html"

Look under buttons for "useful info" and then under
"camera info"

Upon which I asked a noted expert, Henry Scherer, what he thought.

The correct answer is that there isn't any correct answer. The value cited in Peter Tooke's book which is the Zeiss factory specification is an ideal which, in practice, was almost never achieved.

The reason for this is that back in those good old days there was no such thing as any modern method of quality control practiced by anyone in Germany or Russia. In actual practice the actual dimensions of the lens mount as well as the camera body would vary by such a large degree it is my experience that even if one has a camera where the lens flange to film plane measurement is within 35.00mm +0.01/-.03 it is highly unlikely that the rear lens surface will be within the optimal distance from the film plane when it is mounted on the camera.

Zeiss lenses have considerable range of adjustment for the lens cartridge within the lens barrel. And there were a great number of lenses, perhaps most of them, set incorrectly at the factory.

The mounting of the lenses in the rangefinder is also something with a lot of variation. So the rangefinder result has a fair amount of variation in it.

So my overall answer is that it doesn't matter much. There's no point in trying to adjust the position of a lens mount on a camera body when it's likely that the inner machining that sets the position of the lens has a lot of variation.

I think it would have been better for Zeiss to have specified the position of the flanges that are inside the lens mount and to which the lens tabs couple than it was for them to specify the distance from the external tabs. But even if they had done this the variation of everything around the lens mount and which affects the focus is so great this is also rendered relatively meaningless.

There is much truth to be told with this statement. Consider even that if the Film Plane to Outer Flange distance could be held with some remarkable tolerance, the Inner Flange is part of a moving object, the helical itself. Our Jupiter-8 lenses mount to this Inner Flange. The helical cut groove would have to be cut with unimaginable precision to position the Inner Flange at a controlled distance from the film plane. From many measurement I have made it would appear that the difference in distance between the Outer Flange and the Inner Flange is 3.00mm. If 35.00mm is chosen as the Outer Flange to Film Plane distance, then that would make the Inner Flange to Film Plane distance 32.00mm.

June 24, 2007