Stacking two fresnel lens together

Discussion in 'Large Format' started by suk_jung_choo, Dec 5, 2010.

  1. When I purchased my 4x5 camera, I asked the dealer if he had any know hows regarding brigthening the focusing screen . He recommended placing two fresnel ground glasses together. I was skeptical but having little background in large format decided to accept his recommendation and let him install two fresnel ground glasses. My problems are
    1. I am noticing dark rings in the ground glass especially when focusing with the light in certain angles
    2. I have difficulty discerning the correct focus in the periphery (i.e., blurring and double images).
    Could these problems be related to uising two instead of one ground glass? Looking at the pictures that comes back I am not sure if these problems actually affect the photographic end product. However, my suspicion is focusing and composition under the hood are made more difficult by this measure.
    Any comments will be greatly appreciated. Thank you,
    SJ
     
  2. The dark rings are probably due to Moire rings formed by the similar, but not identical patterns in the two Fresnel lenses. Even if they are identical lenses, a small offset between them can cause this. For critical focus, I would recommend inspecting the image on the ground glass with a conventional magnifier.
    Tom M
     
  3. I have 2 fresnel lenses on top of the ground glass in my cambo wide and I experience the same thing as you. The slides and negatives come back bitingly sharp however.
     
  4. Thank you so much for taking the time to respond. Your answers have been very helpful and I have just what I wanted to know. I hope this will be helpful to other members coming across this subject. Regards,
    SJ.
     
  5. There are ground glasses and there are fresnel screens. The two together give the brightest possible and most even illumination. There is no reason why you would use two frenel screens together. You do have a ground glass don't you? Not just two frensnels?
    If your brightest gg and fresnel combination seems too dark then you need a finer gg and, perhaps, a more modern fresnel. But as you get very much brighter you will lose the ability to have the image snap in and out of focus for you. It will be a more gradual difference between in and out of focus as the system losses its "tooth" by extreme brightening. Also, when you do movements it can become more difficult to use a loupe as your eye goes out of the optical axis with extreme brightness screen systems.
    So, assuming that you have a gg and two frenels. Remove one frensnel. See how bright the gg amd fresnel are. Switch to the other fresnel. Is the first combination brighter or the second? Throw which ever fresnel is not as bright out.
    Remember, fresnel screens are a lens, they have a focal length. They are not the focus suface, they exist to distribute the light across the gg not to form the actual image.
    There have been special systems that combined the focusing surface and the fresnel in one piece, an exaample was the Linhof Super Screen. One side was the fresnel and the other side was frosted to form the image. You would put a cover glass on top of the Super Screen to protect the grooves but you would not use an additional fresnel screen.
    There was also the Boss system which used two layers of glass with a layer of wax between them to form the image. This system also did not require a fresnel. But both the Super Screen and the Boss Screen are long gone from production.
     
  6. Bob,
    thank you so much for your detailed explanation. The dealer said that he sandwhiched the ground glass between two identical fresnel lenses. His intentions were honest although perhaps misinformed. Now I think I understand what my problems are. I had a feeling that what you had suggested was infact what I probably needed to do. Since both fresnel lenses were identical products, if I were to use only a single fresnel should it go in front or behind the ground glass? Thank you.
     
  7. Hello !
    If I'm not mistaken, a Fresnel has to be put after the ground glass (as per the light travel path).
    You can put it in front of the ground glass but you've to ensure that the frosted part of the GG still sit where it was before the addition of the Fresnel lens. It is on the frosted surface of the ground glass that the image form. So if you change the ground glass position, you'll change the focusing surface and get blurry pictures (because the film plane is meant to replace exactly the grounded surface of the ground glass...)
    This is why you normally add the Fresnel AFTER the GG, to magnify the already formed image, and not BEFORE the ground glass where it would add another lens of unknown focal length in the light path before forming the image...
    Some camera makers make a combined ground glass and Fresnel lens and they sometimes incorporate the Fresnel lens before the ground glass to protect the delicate rims of the Fresnel inside the camera bellow. But for this to work they have to make calculation and get the calibration of the frosted GG surface right...
    Last but not least, piling up a couple of Fresnel lens is not a bad idea but you should pile them like condensers in an enlarger : plate face facing outward.Keep in mind that the resulting focal length will be the conjugate of the two focal length...
    Hope this helps.
     
  8. The position of the fresnel depends on the design of your camera. Normally, today, the fresnel goes on top of the gg but some cameras were designed for the fresnel to be placed in front of the gg.
    It is critical that you know which way your camera was designed for because the gg ground side must lie at the proper point. By placing a fresnel in front of a gg you may move the gg out of the image plane resulting in your focusing at the wrong place. Some cameras have used both front and rear fresnel placement. For example, very old Technika cameras placed the fresnel under the gg. Current Technikas place the fresnel on top of the gg. Fortunatelty for Linhof owners the camera has adjustable shims so a technician can adjust the position of the gg to allow the fresnel to be placed on top, if desired.
     
  9. The purpose of the Fresnel screen is not to magnify the image, but to collimate the light coming from the GG into a tighter beam. This increases the brightness but narrows the viewing angle, so that the screen actually becomes darker if viewed from off-centre. Stacking two Fresnels will narrow the viewing angle even further, resulting in parts of the screen becoming dark when your eye is moved even slightly away from the focus of the fresnel lenses. Personally I can't see why anyone would want to stack Fresnels, since a good viewing hood or cloth is an easier solution, and the difference in brightness between a plain GG and a fresnel screen isn't that remarkable.
    The most efficient use of a Fresnel is to place it on the inside (lens side) of the GG. However in this position the register of the GG needs to be altered in order to keep the focus accurate. If two Fresnels have been fitted then the only sensible positioning would be to have one on either side of the GG, so I hope your dealer knew exactly what they were doing, otherwise the focus will be out on your camera.
     
  10. "The most efficient use of a Fresnel is to place it on the inside (lens side) of the GG."
    Maybe you would like to explain that reasoning to Linhof and Sinar. Since at least the late 70s Linhof has positioned the Fresnel on top of the groundglass and Sinar has had it there since sometime in the 60s. The reason for that placement is that some people do not want to see enlarged Fresnel grooves when they are focusing with a loupe. So Sinar and Linhof have made systems where the Fresnel is easily removed and replaced when it is not wante. And this also makes gg changing very easy since the thickness of the Fresnel is no longer an issue.
     
  11. Thank you all for responding. I've certainly learned a lot. I will try both ways and see how things go.
     
  12. I think having a double fresnel is simmilar to eating cakes with cakes. :) According to Linhof (Europe) the fresnel should be mounted on the outside of the GG not on the lens side.
     
  13. I went to the dealer and this is what we found.
    The camera we picked was a Linhof technika and the lenses that we tested were the 240 symmar and 90mm super angulon. Removing the outside fresnel made a world of difference in terms of improving resolution and focusing with the 240 mm lens. However, when the 90mm lens was attached under the same circumstances, the peripheral light fall off was almost intolerable .
    Coincidentally, Wista which manufactures the 90 degree reflex view finder with built in fresnel screen clearly specified in the instruction manual, which I read in person, that the reflex finder should be used with the fresnel screen removed for focusing any lens longer than 180mm and above. Perhaps it was for these very reasons that the manufacturer measure recommended by to overcome .
    So maybe the number of fresnel screens used should be adjusted according to the focal length of the lens used. Thanks,
     
  14. "The most efficient use of a Fresnel is to place it on the inside (lens side) of the GG."
    I was referring to the optical efficiency (brightness) of the system Bob, not to the convenience of being able to remove the Fresnel easily. Placing the fresnel on the inside of the GG focuses light from the lens into a more parallel beam before it gets scattered by the GG. If the order is reversed, then some light is lost by scattering before it can be collimated by the fresnel. However there is also, as Suk has now found out, the issue of the Fresnel focal length being more suitable to some taking lenses than to others.
    My own experience with Fresnel screens is that they are of limited usefulness. I have an adapted Graflex back fitted with an inner Fresnel which simply clips into place as an alternative to a plain GG back. This gives me the best of both worlds, as long as I remember to carry the other screen with me.
    Bob, do you know if any manufacturer provides a GG with anti-reflection coating? It's my theory that light reflected from the polished (unground) side of the viewing screen greatly reduces the image contrast, and that if the reflections were reduced the screen would automatically appear much brighter and easier to focus.
     
  15. I have not seen or heard of a coated Fresnel screen. Check with Fresnel Optics to see what they offer. Many camera manufacturers use their screens.
     
  16. Well, based on what I found, I think the idea of using an "adapted Graflex back fitted with an inner Fresnel which simply clips into place as an alternative to a plain GG" as Rodeo commented comes closest to the solution. Compared to all of you, my experience in large format is at best at the beginners level. However, on this issue I think I can say that the perhaps it would be more convenient to adjust the use of the fresnel screens depending on the focal length of the lens used - as wide angle lens tend to show significant peripheral light fall off compared with lens of longer focal length.
    Since this is an open forum, I don't want to add any comments that will cause unnecessary confusion, so I will return after a field test. It's somewhat surprising that a similar topic has not been raised in the past.
     
  17. Than I really don't know why Linhof (the manufacturer of the linhof cameras) states that the fresnel should be mounted outside of the GG. Do anyone has any ideas about that? If it's not a proper way why they saying it? And they arrent mention any size of the lens and they not mentioning any removal of the fresnel either. Simply just that the fresnel should be mounted of the outside of that GG. How abot that?
    Btw. both Sinar and Horseman does that.
     
  18. Frank,
    Both Linhof and Sinar use the fresnel on top of the gg. When fresnels first came out for cameras Linhof placed them under the gg but changed that 40 odd years ago. No one complains about the distribution of light or the brightness or the efficiency of the fresnel on top of the gg. And the benefits are numerous; easy gg change, easy removal of the fresnel when desired, etc.
    There are no benefits that a photographer will derive from the fresnel under the gg save possibly not scratching it from rough handling with a loupe. It is plastic after all.
     
  19. Yes than it's the right way to mount, on the top of the GG (outside) By the way checked my Tachi and the fresnel mounted on the outside too. The top of the GG.
    By the way I don't use loupe, still have a good eyesight and my images are sharp. LoL
     

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