Correct procedure for ground glass placement

Discussion in 'Large Format' started by alpshiker, Jan 25, 2003.

  1. Could you please suggest some ways and tips on how to make sure a new ground glass is correctly registered? It's not that
    much of a problem when the ground glass is exchanged for another plain ground glass, but it becomes difficult when one
    wants to replace for instance an original Fresnel-gg combo where the fresnel in on the lens side, by another system such as
    a Bosscreen (or Beatie, Maxwell...). What I have done so far was set the camera with a luminous lens on and focus on
    something, then replace the set and check if my focus was still accurate. But I'm not sure if this is precise enough. Any better
    way that you could suggest?

    Also on my specific set (Toyo VX-125), the Fresnel-gg combo places the focus plan a little further towards the lens than the
    metal bed where the gg sits (Fresnel on lens side). The other way would have been cared for easily by placing some
    wedges in between, but now I feel that I am stuck. Right?
     
  2. The absolute correct way would be to place a straight piece of drilled plate across the back of the camera where the film holder rests and then taking a depth measurement with a depth micrometer to the ground glass surface. Then place the plate across the film holder and take the same reading to a the septum with a sheet of film in place. Compare the two and see if they agree. If not shim the screen or ground glass to the point of agreement. Using the loupe to see if the focusing point agrees is not accurate enough in my opinion, since it would depend on the aperature of the lens and may be close at one setting but "no cigar" at another. Hope that this helps, good luck.

    Regards,

    Donald Miller
     
  3. Don- Your method is the accepted method when placing a GG only. Any fresnels used would be an "add-on" placed on the outside of the GG(the side towards the photographer). When the fresnel is on the lens side of the GG, all bets are off. The position of these units is compensated to allow for the "frenel focus shift". It's actually called Fresnel Focus Shift Compensation. ( My Arca Swiss is built in this manner.) Neither the fresnel nor the GG will reside at the same plane the film sits at. There are mathematical formulas used to determine the correct placement based on the thickness/lens characteristics of the fresnel. This procedure is best left to facory trained professionals. Steve Grimes may be able to assist.
     
  4. Paul:

    For the accuracy you are going to want, you will have to actually run some photo tests. I use a test rig similar to the one described by Robert Zeichner in his article in the Nov/Dec 1996 issue of View Camera Magazine. Shoot, develop, inspect, shim and shoot again until it is right. Use your fastest lens wide open. If you have to move the gg toward the lens, I guess you will have to do some milling... but that suggests the supplier of the GG hasn't done their job. Are you replacing a Toyo GG with a new Toyo GG, or trying to switch to a different manufacturer?
     
  5. It may not help you in your particular case, but let me relate my experience. I replaced a plain gg in my Toho FC-45X with a Maxwell screen. Bill Maxwell told me to place it just where the old screen went without any adjustments. As best I can tell by testing, my new screen is just where it should be. On the other hand, it is possible the old screen was a little too close to the lens, so there may have been a slight shift after replacing the gg. But if so it was well within any tolerances.

    As others have pointed out, Fresnel lenses facing the lens can cause some problems along these lines.
     
  6. The best way to test focus is to use your widest lens possible. That's were depth of focus is most critical. If you are using a long lens, the difference between focus at 10' compared to focus * may be 15mm of travel on the rail. If your GG is off by 1/20 of an inch, you will never know it.

    On a 47mmXL, the difference between 10' and * may be .5 mm on the rail. A 1/20" error in GG placement here can move your focus from what you though was 50' to maybe 5' out, or perhaps the opposite. Don't rely on depth of field to fix any errors here. If your GG is off, and you are using an ultra wide lens, the problem will certainly show up here.
     
  7. On my last post, * means infinity. My keycaps symbol for infinity came out as an * on this listserve for some odd reason.
     
  8. Some good tips of advice! Yes, Glen, I wished I could replace the Toyo Fresnel-GG by a Bosscreen. Preliminary testing showed a slight focus shift towards the lens. But I might test it again using a more elaborate method such as decribed above to make sure my constatation was not merely due to an inaccurate reading.

    Why a Bosscreen? Well, it's not the brightest but it's the sharpest for critical focussing with a relatively dark image, I used one on the Tech and I really missed it on the Toyo. It has also an advantage over the original GG in that it stays transparent when it is exposed to ambient light, for example when focussing with the loupe in the open (I use a Horseman bino). The Toyo is good as long as it's dark, but when the GG is exposed to light, it becomes like a white screen and you see nothing.

    Thanks for all the good advice. Robert Zeichner mailed me his article and method of testing, thanks Bob.
     
  9. About how to check the position of the gg relative to the film plane.

    It doesn't really matter what focal length lens you use as long as you place the target you are checking on at an appropriate distance relative to the focal length. A distance of about 10 times the focal length should work reasonably well. The main difficulty is focusing as precisely as you can. You do this by using a lens with a large relative aperture when wide open and a high power loupe. Even so, it is hard to be more consistent with an f/5.6 lens to better than 0.3 mm or a little over 0.01 inch. There are a couple of ways to improve your focusing. You can move away from the lens until the target just appears to come into focus and mark the position of the standard on the rail, do the same thing as you move towards the lens, and then note the position halfway in between. You can also focus repeatedly as best you can, note the positions in each case and then take an average of all of them. This is next to impossible to do unless you have a way to mark focusing positions on a fine scale. Often this can be done by marking the position along the focusing knob, since that is usually geared down by a factor close to 5 to 1.
     
  10. BosScreen makes screens for specific cameras rather than a generic screen for all cameras. I was under the impression that the reason for making camera-specific screens was so that the screen could simply replace the ground glass without having to worry about focus shift, shims, etc.

    I've found that the BosScreen is better than a fresnel for fine focusing with a loupe and also for use with wide angle lenses. It doesn't present as bright an image for composing since the screen isn't a "brightening" screen, it just spreads the image out evenly on the ground glass, eliminating the "hot spot" often found with a plain ground glass. This does make it considerably easier to compose but not because the image is any brighter, just because you can easily see the whole image on the ground glass.

    The usual knock on the BosScreen is the fact that it uses wax, which can melt in high temperatures. I used to say that the risk of wax melting in high temperatures was exagerated because I used several in Florida summers without a problem. However, I recently installed a BosScreen in my 8x10 Deardorff. I've started noticing little small bubble-like things in the screen. I see them only when using a loupe, not when composing. The screen still functions just fine but they are mildly annoying. I think they are the result of heat though I'm not 100% certain.
     
  11. Leonard-

    If you'll allow me to indulge myself. I had been using lenses 75mm and longer on my Arca Swiss for several years. Images looked fine. I recently purchased a 47mmXL and found my images were consistantly front focused. I had numerous and lengthy chats on this forum, the ASMP architecture forum, and also with experts such as Rod Klukas at Photomark. It is indeed true that if any of your cameras tolerances relating to GG placement and parallelism are off spec, it will manifest progressively worse as the lens focal length deceases, ie, the shorter the lens, any off tolerances will rear their ugly head. I was advised to continue my testing with my widest lens.

    Rather than test with USAF targets, I use real life situations, after all, I photograph real life scenes. I have a favorite brick building downtown and do my testing at 10:00 full sun. I set up my camera across the street on the same seams on the sidewalk, zero the movements using my Zig-Align, level the camera with a real level, and always aim and focus the camera on one particular area of bricks. There are lamp posts and other elements within my frame at various distances closer to the camera, as well as things further away, approaching infinity focus. When I study the film, I can see if the bricks are sharp, or if these other things closer or further from the camera are sharp.

    I found I had a defective GG assembly and after replacing it, I was finally able to get true sharpness with my wide lens. Again, if I continued to soley rely on mid to longer lenses, the problem with my GG would never have been known.
    I guess this means that images taken with my longer lenses will be even sharper than before :)

    John Luke
     
  12. I just replaced the GG with a Bosscreen on a Linhof 5x7 and as the question of correct placement arose, I made a very simple test that might be helpful to know if you have little time for that. I simply placed a sheet of squared paper vertically on the window and shot it from a 45° stand with a luminous lens wide open (210 f4.5) after focussing on the black line that I had previously marked . The windows should be vertical if the building is well built and by placing the center of the lens, once the camera is set flat, at same height as the center of the sheet, this should be a decent test. The slide taken with the 6x17 back confirmed that the Boss was correctly registered. The images here were taken vertically. This trick should allow for more sophistication as well.
    005xhp-14397684.jpg
     
  13. Test crop
    005xht-14397784.jpg
     

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