Page 2 of 3 FirstFirst 123 LastLast
Results 11 to 20 of 21

Thread: Aligning BOSSCREEN to film plane

  1. #11
    Cor's Avatar
    Join Date
    Nov 2006
    Location
    Leiden, The Netherlands
    Posts
    635

    Re: Aligning BOSSCREEN to film plane

    towolf,

    You used Xray film for your test, right ? Is it a flimsy/thin film ? I ask because you were pointing the camera downwards when shooting. So perhaps the film sagged a bit (bowed out) during exposure, and thus ruining your measurements..

    best,

    Cor

  2. #12

    Join Date
    May 2011
    Posts
    158

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Cor View Post
    You used Xray film for your test, right ? Is it a flimsy/thin film ? I ask because you were pointing the camera downwards when shooting. So perhaps the film sagged a bit (bowed out) during exposure, and thus ruining your measurements.
    This is also very good point. I was fretting about it already. Film flatness. The film is rigid and 0.2mm thick (on the thick end of the sheet film spectrum, and with double emulsion). But I have some doubts about it. I will repeat the film tests with carefully checked flatness. Maybe with a small patch of double-sided tape in the center.

    So the general suspicion I have is that the 45į newspaper test was probably confounded and the measurement with the microscope was likely (more) accurate.

  3. #13

    Join Date
    Sep 1998
    Posts
    10,336

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by towolf View Post
    I can ask, but the last time I wrote email to linhof.de they quoted 25 EUR for an infinity stop. Iíd rather figure this out myself. That is half of the fun in LF for me.



    No, it says BOSSCREEN HOLLAND - PLAUBEL. Are there differences in the stand-off?

    In the end I donít want to buy anything or pay someone. I just want to know the best method to co-register the screen I have.
    Yes, that is why yours says Plaubel. If it was the one that they made for Linhof it would say Linhof.

    And, again, the best way is to put the camera back together properly. Then adjust the gg to the correct Linhof position.

    As for the 25 EUR. Was that for one or for the pair with the set screws?

  4. #14
    Maris Rusis's Avatar
    Join Date
    May 2006
    Location
    Noosa, Australia.
    Posts
    972

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by towolf View Post
    That is an interesting point. However, didnít 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.
    Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".

  5. #15

    Join Date
    Sep 1998
    Posts
    10,336

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Maris Rusis View Post
    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.

  6. #16

    Join Date
    May 2011
    Posts
    158

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Maris Rusis View Post
    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.

  7. #17

    Join Date
    Jun 2006
    Location
    San Diego, CA
    Posts
    205

    Re: Aligning BOSSCREEN to film plane

    That is common problem for focusing screens that that have extra glass surface in front of focusing surface. This was discussed here many times, short summary here:

    http://www.largeformatphotography.info/gg.html

    Mainly extra glass needs to be compensated by "about" 1/3 of its thickness. But "about" depends on lens focal lenght, glass properties etc. that was not problem with film and moderately long 4x5 lenses. Arca used similar design with fresnel in front of GG.

    Once high res digital backs become standard (well not on this site) and very short focal lenght become standard problem turned out real pain to deal with. Arca introduced N type backs etc, Linhof has gazzilions of focusing screen revisions, but problem was never solved completely. Sinar always used GG and had no problems.

    Modern digital is focused on computer screen via live view and that is not problem anymore.

    Overall with 4x5 film it is not real problem - film is art and not pixel pocking. Focusing gear pinion on your camera has bigger positioning error than 0.3mm. If this is is really annoying for you get plain GG.

  8. #18
    Maris Rusis's Avatar
    Join Date
    May 2006
    Location
    Noosa, Australia.
    Posts
    972

    Re: Aligning BOSSCREEN to film plane

    VictoriaPerelet has the story right on Bosscreens and other configurations. Just for fun here are some numbers:

    If the wax layer is covered by soda lime float glass (my guess) with a refractive index of 1.523 (my guess) and a thickness of 2mm (my guess) then the Bosscreen has to be shimmed back 0.688 mm so that the lens "sees" the wax layer at the same optical distance as the film. Different guesses calculate out to different shim thicknesses. All this is ok for paraxial rays only. For rays arriving obliquely, say at the edges of the sreeen from a wide angle lens, those rays have a greater pathlength through the coverglass. This results in image shift that gets worse the more the rays are off-axis. I've heard this phenomenon called "spherical aberration in the image space". I doubt that ordinary view camera work with real world cameras and subject matter will encounter these curious difficulties.
    Last edited by Maris Rusis; 27-Oct-2014 at 22:51. Reason: spelling
    Photography:first utterance. Sir John Herschel, 14 March 1839 at the Royal Society. "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..".

  9. #19

    Join Date
    Sep 1998
    Posts
    10,336

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by VictoriaPerelet View Post
    That is common problem for focusing screens that that have extra glass surface in front of focusing surface. This was discussed here many times, short summary here:

    http://www.largeformatphotography.info/gg.html

    Mainly extra glass needs to be compensated by "about" 1/3 of its thickness. But "about" depends on lens focal lenght, glass properties etc. that was not problem with film and moderately long 4x5 lenses. Arca used similar design with fresnel in front of GG.

    Once high res digital backs become standard (well not on this site) and very short focal lenght become standard problem turned out real pain to deal with. Arca introduced N type backs etc, Linhof has gazzilions of focusing screen revisions, but problem was never solved completely. Sinar always used GG and had no problems.

    Modern digital is focused on computer screen via live view and that is not problem anymore.

    Overall with 4x5 film it is not real problem - film is art and not pixel pocking. Focusing gear pinion on your camera has bigger positioning error than 0.3mm. If this is is really annoying for you get plain GG.
    Linhof revisions are simply screens with different rulings on them or screens of much much brighter output and both types have exactly the same placement in exactly the same holder. They do not change with the type of back used unless the new back has a different sized sensor.

    A better example would be Rodenstock using the glass cover plate as the last element of the lens with some of their digital lens series and selling an optional glass plate that is used when those lenses are used with film.

  10. #20

    Join Date
    May 2011
    Posts
    158

    Re: Aligning BOSSCREEN to film plane

    Quote Originally Posted by Maris Rusis View Post
    All this is ok for paraxial rays only. For rays arriving obliquely, say at the edges of the sreeen from a wide angle lens, those rays have a greater pathlength through the coverglass. This results in image shift that gets worse the more the rays are off-axis. I've heard this phenomenon called "spherical aberration in the image space". I doubt that ordinary view camera work with real world cameras and subject matter will encounter these curious difficulties.
    So, would you say my opcitcal method of establishing the shimming position is going to lead to more-or-less correct results?

    I already noticed that when I use my microscope focussing method through the front or through the back I get a difference of 0.36mm. The rear glass (facing the user) is quite a bit thicker, so that would explain it.

Similar Threads

  1. Aligning Omega D5 and Beseler 4x5
    By neil poulsen in forum Darkroom: Equipment
    Replies: 23
    Last Post: 1-Oct-2010, 06:05
  2. Aligning Layers in Photoshop
    By neil poulsen in forum Digital Processing
    Replies: 7
    Last Post: 23-Jan-2009, 14:04
  3. Aligning a Chromega F
    By Bruce Schultz in forum Darkroom: Equipment
    Replies: 8
    Last Post: 11-May-2006, 18:29
  4. aligning my enlarger
    By brian steinberger in forum Darkroom: Film, Processing & Printing
    Replies: 7
    Last Post: 20-Mar-2006, 23:55
  5. aligning enlarger
    By brian steinberger in forum Darkroom: Equipment
    Replies: 10
    Last Post: 21-Aug-2005, 19:59

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •