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Thread: Fresnel screen focus distance is different from ground glass

  1. #11

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by Bob Salomon View Post
    A Fresnel does 2 things:
    1 brightens the gg image
    2 even outs the illumination on the gg edge to edge.

    Ask the manufacturer how they propose that you do these 2 things?
    Or
    What they will do to your camera to achieve those 2 things?
    Bob,

    The Chamonix N-1 was designed poorly; the addition of the Fresnel lens was not taken into account and adding it, while brightening the image and evening the illumination, threw the focus off. The easy fix is to remove the Fresnel and use the camera with the ground glass only, similar to many other models of view camera that do not come with a Fresnel. Sure, the image is a bit dimmer, but focusing is more precise. Many prefer the standard ground glass to a screen with a Fresnel lens added.

    Best,

    Doremus

  2. #12

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    Re: Fresnel screen focus distance is different from ground glass

    Originally I wasn't aware the issue is specific to this camera model so I was looking for a general confirmation of whether this is not an expected behavior. Turns out this topic has been discussed here quite a lot so I don't think I can add much to it at this point. The issue was fixed in the revised model which is commendable. I'm fine with using the camera without fresnel as I wasn't expecting one to be there in the first place.

  3. #13

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by Doremus Scudder View Post
    Bob,

    The Chamonix N-1 was designed poorly; the addition of the Fresnel lens was not taken into account and adding it, while brightening the image and evening the illumination, threw the focus off. The easy fix is to remove the Fresnel and use the camera with the ground glass only, similar to many other models of view camera that do not come with a Fresnel. Sure, the image is a bit dimmer, but focusing is more precise. Many prefer the standard ground glass to a screen with a Fresnel lens added.

    Best,

    Doremus
    Or mount the fresnel, groove side down, on top of the gg.

    The factory should be easily able to provide a means of doing this, it only involves a clip on each end of the gg.

  4. #14

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    Re: Fresnel screen focus distance is different from ground glass

    In any case after you have made your modifications check for sure with real film.
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

  5. #15

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by cowanw View Post
    In any case after you have made your modifications check for sure with real film.
    If you mount the fresnel on top of the gg there is no change to the positioning of the ground side of your gg. So focusing will be just as accurate as long as you focus your loupe on the ground side of the gg.

  6. #16

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    Re: Fresnel screen focus distance is different from ground glass

    Won't the GG be allowed to fall forward to the position where the Fresnel is presently? And be a few mm closer to the lens position? Or is the lens held in position even if the space the present Fresnel occupies is vacant.
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

  7. #17

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by cowanw View Post
    Won't the GG be allowed to fall forward to the position where the Fresnel is presently? And be a few mm closer to the lens position? Or is the lens held in position even if the space the present Fresnel occupies is vacant.
    Not if you have his camera that was never designed to have anything, other then a film holder, under the gg!

  8. #18

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by Bob Salomon View Post
    Not if you have his camera that was never designed to have anything, other then a film holder, under the gg!
    Well, despite the fact it wasn't designed for fresnel screen, there are two separate steps in the back wall, a pair of vertical and a pair of horizontal. The horizontal ones are deeper than vertical. The fresnel rests on the horizontal and does not affect the position of the ground glass which rests on the vertical steps. So yes, moving the fresnel to the other side of the ground glass works, but the fresnel needs to be fixed in its new place in some way for which there is no ready-made provision.

  9. #19

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    Re: Fresnel screen focus distance is different from ground glass

    Quote Originally Posted by nicnilov View Post
    Well, despite the fact it wasn't designed for fresnel screen, there are two separate steps in the back wall, a pair of vertical and a pair of horizontal. The horizontal ones are deeper than vertical. The fresnel rests on the horizontal and does not affect the position of the ground glass which rests on the vertical steps. So yes, moving the fresnel to the other side of the ground glass works, but the fresnel needs to be fixed in its new place in some way for which there is no ready-made provision.
    Not difficult, Linhof added a simple rectangular bar to the middle gg hold down bar screw. Push them to one side, drop the Fresnel on top of the gg and swing them back. Very simple and works extremely well!

    http://linhof.com/wp-content/uploads...x5-600x734.jpg

  10. #20

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    Re: Fresnel screen focus distance is different from ground glass

    Here is an interesting thread, that seems to answer the question.
    https://www.largeformatphotography.i...focusing-error
    I particularly like Bob's answer then on page 3.
    "If a Fresnel, under a gg is causing incorrect focus then the gg position is incorrect. Simply placing the Fresnel on top of the gg or throwing the Fresnel away will not be an automatic cure as someone earlier noted, the thickness of the Fresnel has an effect on focus as any piece of glass or plastic in the optical path shifts the focus by about 1/3rd the thickness of whatever is placed there.

    The first thing that you want to check is the focus on and inclined yard stick, with the Fresnel and gg in the factory position. Put a vertical flag pin on the yardstick at the point you are focusing on and focus with a loupe of 4 to 6x power whose eyepiece is adjusted to bring the grain of the gg into sharp focus. Focus on the point where the flag pin is inserted into the ruler. Make an exposure wide open and at f22. Make sure that the lens is within the optimization range for the magnification you are using. Testing with a lens corrected for 1:20 at this range won't tell you much as the lens will not perform optimally.
    What is the result? The focus spread on either side of the flag pin will tell you what is happening."
    I still think testing it now would at least be reassuring.
    Bill
    "There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).

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