Fresnel Lens combo w/gridded gg issue

Discussion in 'Large Format Cameras and Accessories' started by djkloss, Dec 6, 2017.

  1. djkloss

    djkloss Subscriber

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    I just installed a new plastic gg with grid (mat side towards lens) then the fresnel (w/grooves toward lens) - according to the YouTube Video I watched :wink: I like how bright it is, but the annoying thing about it is the light coming through the camera from the lens. It looks like a giant full moon and makes it very hard to focus. Other than that, I like it. Is that because it's a cheaper brand? I got it on ebay several years ago and the sticker says Global Screen. The man's name I think is Mr. Wang. Any thoughts would be greatly appreciated!
     
  2. shutterfinger

    shutterfinger Subscriber

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    What camera did you install the combo in?
    Have you tried the plastic gg on its own to see if the image is sharp?
     
  3. AgX

    AgX Member

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    Any field lens, and the fresnel lens acts as such, forms part of the complete optical system of your camera and as such must fit to the very situation.
     
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    djkloss

    djkloss Subscriber

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    It' a Kodak 8x10 2D. No I haven't tried it on it's own, but I tried it with the original gg, which worked ok. I was wondering if the new gg was too thin or something.
     
  5. shutterfinger

    shutterfinger Subscriber

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    The ground/matt side of any focus screen/ground glass should face the lens, if not the focus will be off on any film used in the camera.
    I would start by trying the focus screen on its own as that is likely where the problem is.

    The focus panel on that camera is made so that the ground side/matt side of a ground glass/focus screen sits at the same position as a sheet of film will be when inserted into the camera. The thickness of the ground glass or focus screen nor a fresnel used behind the ground glass/focus screen will affect focus of the camera but the retainers may have to be modified to hold thinner or thicker screens/combinations in place.
     
  6. shutterfinger

    shutterfinger Subscriber

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    The most common reason to change the factory ground glass is to improve brightness/ease of focusing.
    A fresnel dims bright spots and and improves seeing into the shadows, it does not increase overall brightness.

    The most common problem with older ground glass is they become dirty and pick up a dulling haze layer that does not wipe off with standard cleaning.
    1. Remove the ground glass from the camera.
    2. In a pan of warm water immerse the gg then apply a drop of dish detergent (I use Dawn) about the size of a fingernail (base to tip of the finger) directly to the glass.
    3. Gently rub the glass with your finger tips only while holding by the edges.
    4. Rinse and repeat for the other side.
    5. Rinse thoroughly under running water then rinse in a weak PhotoFlo solution.
    6. Stand on edge and allow to air dry or dry with a clean lint free cloth.
    Average improvement is 1 to 2 stops of brightness.
     
  7. Doremus Scudder

    Doremus Scudder Member

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    It sounds as if you've installed everything correctly, but you're experiencing a "hot spot," i.e., a bright circle of the image in the middle of the ground glass and darker corners.

    If that is correct, it may simply be that the focal length of the lens you are using in conjunction with the Fresnel screen causes this. This phenomenon is fairly common when using a shorter focal length lens along with the Fresnel. If your lens is in the 200mm or less range on 8x10, this is likely the cause. Try it with a longer lens and see what happens.

    Another possibility is that the focal length of the Fresnel is simply not matched to the "normal" for 8x10. Many people don't realize that a Fresnel screen is a lens and has a focal length. In order to avoid the hot spot, the focal length of the Fresnel should match the focal length of the lens being used. In practical application, Fresnel screens are matched to a middle value, usually the "normal" focal length for the format being used, which in the case of 8x10 is around 300mm. Maybe yours simply isn't...

    Best,

    Doremus
     
  8. Marco Gilardetti

    Marco Gilardetti Member

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    I'm a bit surprised but I have to disagree with the above posts. If I got what you've written correctly, djkloss, your groundglass and fresnel lens are not installed properly. The matted side of the groundglass and the grooved side of the Fresnel lens should be in contact with each other. If they're not in contact with each other, the image will form on the groundglass and on the grooves of the Fresnel lens as well, and you will have a very hard time focusing. Also, in general (I've never owned your camera specifically), if the mat side comes first and it's oriented towards the lens, then it will not be on the same plane of the sheet film, and your negatives will be defocused.

    Given that the two machined surfaces touch each other and that the glass (or plastic) of both has the same thickness, it's not important which of the two is placed towards the lens as the center of the "sandwich" will be placed right in the middle anyway (and in the position in which the sheet film will be). You can try both orientations and see if one of the two works better with you, in my experience it makes no difference (I ended up with the groundglass plate towards the lens and the fresnel plate towards me because there are writings on my grid and in that way the writings were left-to-right, but as said optically I didn't notice any difference the other way around).

    Moreover, in my experience the Fresnel lens improves greatly the overall brightness of the groundglass. In every camera in which I've tried to do without, I had to re-install it shortly thereafter.
     
  9. shutterfinger

    shutterfinger Subscriber

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    Marco, you are giving false, misleading information on camera focusing panel set up. Go take a basic course in camera set up/repair.
    Your comments dispute Folmer and Schwing/Kodak, makers of the camera, technical information about the camera which agrees with many other camera manufacturers technical information.
    The ground side of the ground glass ALWAYS faces the lens/front standard.
    The ribbed side of the fresnel ALWAYS faces the ground glass regardless of which side of the ground glass it is placed.
    If the fresnel is placed in front of the ground glass and its sits on the same mounts as the ground glass then the mount must be machined to compensate for the focal length of the fresnel.

    Doremus, I had a Sinar F1 that had a fresnel on the user side of the ground glass. I used a 90mm Super Angulon to a 300mm Dagor with no blooming effects as you describe. It was a factory fresnel and I do not know its optical focal length.
    I sold it and bought the 8x10 Kodak Commercial View in my avatar.

    djkloss lets us know what you find.
     
    Last edited: Dec 7, 2017
  10. Doremus Scudder

    Doremus Scudder Member

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    shutterfinger,

    All my Fresnel screens, i.e., factory screens on Wista DXs, Wista SWs, Graphic View II, etc. have a "hot-spot" problem with shorter lenses, particularly with 90mm and shorter (this on 4x5 format). These all are factory sandwiches and appear to have the frosted side of the ground "glass" (Wista screens are plastic) facing the lens. The problem can be ameliorated by finding the optimum viewing distance. This doesn't bother me much, since the hot spot moves around with the viewing position, so focusing at any particular spot on the ground glass is easy; it's just the overall view that gets dark in the corners. Maybe I should look for a Sinar ground glass :smile:

    Doremus
     
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    djkloss

    djkloss Subscriber

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    Thanks for all your responses, greatly appreciated! Well, the lens I'm using is a Kodak 14" Commercial Ektar f/6.3. I have tried all the different line-ups. And have concluded...in my very humble opinion, the plastic ground glass is not etched enough. It's clear enough that you can see through it fairly easily, thus allowing the light to come through the lens creating a big full moon. I can see to focus the subject just fine, but the moon gets distracting. End result, I cleaned the original gg and put it back together the way it's been working for all these years. The reason I wanted to try the other gg, was because of the grid. But, I think I'll just stick to the old. As they say, if it ain't broke, don't fix it! :smile: Again thank you for all your feed back! Much obliged!
     
  12. Dan Fromm

    Dan Fromm Member

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    dj, if you want a grid, a #000 micron pen and a ruler will make one. Micron is a brand.
     
  13. shutterfinger

    shutterfinger Subscriber

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  14. OP
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    djkloss

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    Thanks, I wonder if they make it in silver or gray, since my backgrounds are usually black. thanks for the tip
     
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    djkloss

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  16. Marco Gilardetti

    Marco Gilardetti Member

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    If I overfly for a while the unbearable conceited, professorial tone of your reply, I find it ironical and amusing because - I candidly admit it - I had basically recopied what I have written from the renowned book by Leslie Stroebel "View Camera Technique", 7th edition, pages 256-260, figures 12-11, 12-12, 12-13 and 12-14. The irony is increased by the fact that, in your subsequent academic lecture, you repeat what I had just written. Since every day is good to learn something new for people who have an open mind, I would then like to understand, in detail, what you've found so outrageous in my post and, seemingly, in what Leslie Strobel writes.
     
    Last edited: Dec 11, 2017
  17. Ian Grant

    Ian Grant Subscriber

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    The Wista screens are exceptionally good in terms of brightness, the improvement over a plain screen is substantial with 150mm and longer and I don't find it an issue with my 90mm or 75mm, It's the same combination screen and the brightest of all my 5x4 cameras with fresnels. I actually find it harder using a plain screen very much more of a hot spot that doesn't move with your eye.

    One of my MPP Micro Technical cameras, a MkVII, has a plain Sinar screen, I'll probably add a fresnel to the rear of the gg (eye side). I did this a few years ago to a Crown Graphic and along with a new screen added 3 stops effective brightness, should add all the Graflex screens were quite poor.

    Ian
     
  18. OP
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    djkloss

    djkloss Subscriber

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    Thanks Ian, Does Beatty still make screens?
     
  19. Ian Grant

    Ian Grant Subscriber

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    Beattie Intenscreens :D Well the website is still up and running listing distributors etc and the UK one has some in stock for MF cameras and some 5x4. Last time I looked they had 10x8 screens but none listed any longer.
    I have one on my main Agfa Ansco 10x8, a Commercial View, it was thrown in unused (NIB) along with a Dagor, 3 new lens boards, a Compendium hood etc. I know it was worth about 50% of what I paid for the camera at the time, they are expensive.

    To be honest I've been very happy with the regular fresnels on my Crown, Speed and Super Graphics, all have had new GG (which made the greatest difference) and now they are about 1/2 a stop less apparent brightness compared to my Wista screen and I can work with them very easily.

    Ian
     
  20. Ian Grant

    Ian Grant Subscriber

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    Just a comment about the original seller I think you bought from. I had a very bad experience about 9 or 10 years ago with him on Ebay, the screen didn't arrive despite being tracked, nor did a second (which he couldn't prove hed posted) a third many months later was obviously not brand new but by his delaying tactics it was too late to make a claim through ebay or Paypal. Ebay actually closed his account as there were so many complaints and there was discussion on this and other forums.

    His current website has the same photos etc that he used on it and ebay when I bought from him :D

    Ian
     
  21. Mr Bill

    Mr Bill Member

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    These two posts are exactly right, and are the key to understanding this. Except that it is not necessarily correct that the Fresnel lens fl should (always) match the fl of the camera lens, although this is generally a "safe" thing to do. The idea is that the Fresnel (being used as a "field lens") should project an image of the camera lens' opening (its pupil) into the pupil of the viewing system (your eye). So it also depends on how far the lens is "racked out" and where you intend to have your eye. In the typical situation that you are doing scenic photography (the camera lens is only racked out a small amount) and you want to be able to use a loupe on the GG while looking directly at the surface, then (roughly) matched fl is good.

    And the fact that a ground glass scatters the light makes the fl choice more forgiving.

    If I were you I would take the bare Fresnel lens out in the sunlight and get an approximate fl (do this by focusing the sunlight onto the ground (you might want to wear sunglasses or mask off a large part of the lens as it will be very bright; it will also try to burn things up right away), then measure (roughly) the distance from Fresnel to ground. This will give you a good idea of how different your camera lens and Fresnel differ in fl.

    Photographers don't generally need to know how this works, but if you are the sort who is occasionally stymied by this sort of thing then a couple of hours sketching lighting rays via the standard lens formulas can lead to an understanding.
     
  22. EdSawyer

    EdSawyer Member

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    agreed, it's a mismatch on the focal length of the screen. I have seen this with cheap fresnels from ebay.
     
  23. AgX

    AgX Member

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    The issue is similar to the condensor issue at point-light enlargers.
     
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    djkloss

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    all very good information, thank you all!