Aligning BOSSCREEN to film plane

[towolf]

I’m looking to reconcile two contradictory findings in my pseudo-scientific endeavor to nail down focus on the BOSSCREEN ground glass that my camera came with.

I’m presenting my method and if someone can spot my mistake I would be very grateful.

I was shooting slides (E100G) for the first time and was just a little disappointed with the detail on the horizon of a landscape scene. The shot was made racked against the infinity stop of a Linhof Tech V. Now, I have checked the infinity stop against the GG painstakingly in my endeavor of projecting a laser through the rangefinder. My new paranoid hunch was that maybe the GG was not co-registered with the film plane. Now, the previous owner cannot be asked how he installed it, because he fell victim to cancer (which is why I have the camera).

Since I had a big load of old Xray film I commenced to photograph a table top with a newspaper on it at 45°



I focussed on a particular line (thin line under "Asset Management" at 10cm on the paper ruler) but the focus on the developed sheet seemed to be 1 cm out.

Now, 3 columns of text were (roughly) 18cm wide on the paper and 7cm on the film giving a magnification of 1/2.6. The other calc 150/(545-150) found the same ratio of 2.6.

I calculated with 10mm offset "(10mm * (sqrt(2)/2)) / (2.6**2) = 1mm", and hence got 1mm film deviance from the ground glass.



Also in the repeat.



Then I lifted the BOSSCREEN from its frame and found no metal shims (which are supposed to be there on a Technika) but two 0.9mm thick rubber-like stacks of electrical tape instead of shims.

[towolf]

Since the 0.9 mm were agreeing with my estimate I peeled them off and fixed the screen directly onto the metal flange with the four set screws fully retracted. I took another test shot and focus seemed to match now.



However, and now comes the problem, today I took more test shots comparing the old infinity stops to my new ground glass setting but things at infinity didn’t get better, they got worse. For some reason what matched at close-up distance didn’t match at infinity.


So, today I wanted to try a different route to measure the distance of the film plane and Bosscreen wax layer each. Just breaking out the calipers would not have given the right accuracy obviously. And I have no advanced tools like micrometers. And in the BOSSCREEN the image layer is in the center of a glass sandwich preventing purely mechanical measurements.

But, I have also inherited an old 40x Zeiss stereo microscope:



I dropped a Durst enlarger glass on top of the GG frame to measure the focus difference between a felt pen mark on the underside of the glass pane



and the wax layer inside the screen.

[towolf]

Then I could use my callipers to measure from these two defined spots




to make a measurement with ±0.05mm accuracy (by reading the nonius at right angles)

[towolf]

The vertical focus offset between the lower and the upper level would give me the flange distance from the GG




And the same story with the film holder




I measured everything 3 times and calculated both flange distances and got 4.8mm for the film holder (the specification standard seems to be 3/16 inch, so 4.76mm) and got 3.9mm for the new "naked" position of the BOSSCREEN wax layer. A difference of 0.9mm!!!

So it appears, after all, that the original rubber shims were correct when measuring (opto-)mechanically. But on my table top newspaper test there was a clear problem.

If anyone of the wonderfully knowledgeable people here can spot my error please help me find it.

[Maris Rusis]

It could be an optical path difference problem caused by the glass in front of the wax layer. Light passing through glass appears to travel a greater distance than if there was only air there. Optical pathlength is geometric pathlength multiplied by the refractive index of the medium the light is passing through.

[towolf]

It could be an optical path difference problem caused by the glass in front of the wax layer. Light passing through glass appears to travel a greater distance than if there was only air there. Optical pathlength is geometric pathlength multiplied by the refractive index of the medium the light is passing through.

That is an interesting point. However, didn’t I already compensate for this by measuring through the glass in question? Or does the refraction by the inner pane of glass depend on the incident angles? Does this explain the difference between 0.75 m and infinity?

That would be a flaw in the Bosscreen itself, no less.

[Maris Rusis]

That is an interesting point. However, didn’t I already compensate for this by measuring through the glass in question? Or does the refraction by the inner pane of glass depend on the incident angles? Does this explain the difference between 0.75 m and infinity?

That would be a flaw in the Bosscreen itself, no less.

You can't measure where the wax layer in a Bosscreen is by focussing on it through the cover glass. All you get is where it appears to be because of the optical path length effect of the cover glass.

This is an easy test to do. Set up a view camera with the bellows off and exactly focus a subject. Now place a sheet of glass between lens and ground glass and you will notice the subject is now out of focus. To regain focus the length of the camera must be extended.

The wax layer of a Bosscreen cannot be placed by geometric measurement so as to coincide with the film plane in a film holder of correct T distance. It has to be placed optically so that when an image is sharp on the film it is also sharp on the wax. Because the exact position of the screen is influenced by the thickness and refractive index of the glass between the wax and the lens correct shim thickness is found by trial and error.

[Bob Salomon]

You can't measure where the wax layer in a Bosscreen is by focussing on it through the cover glass. All you get is where it appears to be because of the optical path length effect of the cover glass.

This is an easy test to do. Set up a view camera with the bellows off and exactly focus a subject. Now place a sheet of glass between lens and ground glass and you will notice the subject is now out of focus. To regain focus the length of the camera must be extended.

The wax layer of a Bosscreen cannot be placed by geometric measurement so as to coincide with the film plane in a film holder of correct T distance. It has to be placed optically so that when an image is sharp on the film it is also sharp on the wax. Because the exact position of the screen is influenced by the thickness and refractive index of the glass between the wax and the lens correct shim thickness is found by trial and error.

The Linhof shims are set to factory specs on the screws that they sit on. The correct Boss screen for the Linhof has the bottom layer of glass cut off where the glass would sit on the shims so the wax layer is then at the correct position. Like the MAster Technika all V, Master Technika, Master Technika 2000 and Master Technika 3000 cameras have the gg position zeroed at the factory to the same position. So then it is simply child's play to change the gg and have it at the correct film plane position.

However, you can not assume that the bottom glass layer on the Linhof version of the Boss screen is the same thickness as the ones for other model cameras.

So, you know the proper procedure. The first step is to get the gg shim position at the correct position. Then modify the Boss Screen or get the correct one.

[towolf]

You can't measure where the wax layer in a Bosscreen is by focussing on it through the cover glass. All you get is where it appears to be because of the optical path length effect of the cover glass.

This is an easy test to do. Set up a view camera with the bellows off and exactly focus a subject. Now place a sheet of glass between lens and ground glass and you will notice the subject is now out of focus. To regain focus the length of the camera must be extended.

Yes, I’m wondering about that. But consider this:

You made two marks for focus positions between a close and a far point. Let’s say 5 mm without the glass. Now you insert your sheet of glass, but you also move the GG farther away until the subject is *again* in focus. The scale will still read 5 mm.

So with my setup, if I manage to keep the focus of my microscope at 4.8 mm and shim the Bossscreen appropriately so that it is in focus at 4.8 mm (even though the wax is actually farther away) I will have correctly shimmed the GG.

I made a quick visualization

The wax layer of a Bosscreen cannot be placed by geometric measurement so as to coincide with the film plane in a film holder of correct T distance. It has to be placed optically so that when an image is sharp on the film it is also sharp on the wax. Because the exact position of the screen is influenced by the thickness and refractive index of the glass between the wax and the lens correct shim thickness is found by trial and error.

So much is clear, which is why I didn’t measure purely mechanically but with the help of finely adjustable optics.

[Bob Salomon]

First, contact a Linhof service center and get a set of the correct shims. They are the same as the current ones for the MT.
Second, Do you have the Boss Screen for the Linhof?

If not, find one, if you can or just get a new enhanced screen from anyone else.