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Thread: What is a fresnel lens?

  1. #1
    Lucian Marin
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    What is a fresnel lens?

    I have been using LF for a couple of years now, but still am having trouble understanding what do some photographers use fresnels for?

    I did a lot of searching around, but all i can find is stuff related to its placement, in front or behind the GG, but nobody seems to explain why they use it. What is the purpose?

    Or is that fresnel screen? Is there a difference? What do these things even look like?

    Kindly appreciated.
    Thanks!

  2. #2
    8x10, 5x7, 4x5, et al Leigh's Avatar
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    Re: What is a fresnel lens?

    A fresnel lens is a regular plano-convex lens that's been reduced in thickness across its width.

    This is done by profiling the angle of the convex face as a series of concentric rings, each with the surface angle corresponding to that portion of a real lens.

    Fresnel lenses are commonly used to increase the apparent brightness of a ground glass image by focusing the light toward the viewer.

    - Leigh
    If you believe you can, or you believe you can't... you're right.

  3. #3

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    Re: What is a fresnel lens?

    Hello from France

    The name comes from the famous French physicist Augustin Fresnel, one of the pioneers in optics in the XIX-st century. He designed a famous lens system for lighthouses. He proved that you could efficiently focus light by using only a fractionof the weight of a positive glass lens element.

    A Fresnel lens acts exactly like a thin positive lens element of very large diameter.
    You can design a Fresnel lens with a diameter bigger than to its focal length. With a classical glass lens this would be either impossible or impractical.

    Fresnel lens are added to ground glasses in many cameras where you can have access to an optical image.
    The purpose of the Fresnel lens is to bend slanted rays toward your eyes so that the corners of the ground glass appear as bright as the centre of the field.
    On a large format view camera or a field camera, the Fresnel lens is very useful for standard and wide-angle lenses. For long focal lengths, it is less useful.

    The following diagram explains why the corners of the field appear dark on a ground glass, it is simply because the light diffused by the ground glass somewhat "remembers" the direction of incident light rays emitted by the lens.
    The Fresnel lens will simply bent rays re-emitted by the corners of the ground glass toward the photographer's eyes.

    The diagram also explains why the effect is more important for wide-angle lenses that for long focal lengths.
    More precisely, what matters is the distance between the position of the exit pupil of the lens and the position of the ground glass.
    This distance is not always equal to the focal length of the lens in use !!
    It can be much bigger when you focus in the close-up range.
    It is close to the focal length for a quasi-symmetrical lens deisign focused for infinity
    And it can be much shorter than the focal length for a telephotolens focused for infinity.

    Most, if not all, Fresnel lenses used in large format cameras are made of molded plastic and their focal length is roughly equal to the diagonal of the ground glass, i.e. the diagonal of the format e.g. 150 mm in 4x5".

  4. #4

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    Re: What is a fresnel lens?

    Emmanuel,

    Great explanation.

    Did you know that Raymond Blanc was just home for his parents' birthdays (Nov. 11, 1921 and Nov. 11, 1922)? Report with photos on his blog, looks like everyone ate and drank very well: http://www.raymondblanc.com/BLOGS/Ma...ebrations.aspx
    Cheers!

  5. #5
    Andrew's Avatar
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    Re: What is a fresnel lens?

    apologies fro the low res GG shots but it seemed an easy way to make the comparison... digicam pointed at 4x5 GG with 65mm SA focused to infinity.
    Pics with and without the fresnel but digicam set to manual so there's no changes induced by the camera trying to correct the exposure.
    Should be pretty obvious that you get a brighter peripheral image with the fresnel.

  6. #6
    Lucian Marin
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    Re: What is a fresnel lens?

    Thanks Andrew. Related to the fresnel's position, on which side of the GG would you use it? I can read some people place it between the lens and GG, which apparently can affect focusing, while others use it outside the GG, photographer side. Which is best?

  7. #7

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    Re: What is a fresnel lens?

    Oooooh. I always thought "Fresnel" was the French version of "Ansel", and a Fresnel Lens was a special lens that made all of your images as good as Moonrise Over Hernandez.

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    Re: What is a fresnel lens?

    Quote Originally Posted by lmlmlm View Post
    Thanks Andrew. Related to the fresnel's position, on which side of the GG would you use it? I can read some people place it between the lens and GG, which apparently can affect focusing, while others use it outside the GG, photographer side. Which is best?
    Best in terms of even illumination is between lens and ground glass, matched and adjusted for the focal length of the lens.

    But in almost every other aspect it is more problematic. That arrangement causes a difference between the ground glass and film focal planes, so that it has focus shift and illumination issues with other focal lengths than the one it is adjusted for. Besides, fresnel lenses don't work too well with extreme movements (where the rays are far from perpendicular), so you have to be able to temporarily remove the fresnel on cameras capable of more considerable movements - which is only possible with a behind-the-GG configuration.

    That is, behind the ground glass is the standard configuration on optical bench and technical cameras, while press and travel cameras, or indeed anything with limited movements and lens change capability, can use the (brighter, but more restricting) before the GG configuration.

  9. #9
    Andrew's Avatar
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    Re: What is a fresnel lens?

    Quote Originally Posted by lmlmlm View Post
    Thanks Andrew. Related to the fresnel's position, on which side of the GG would you use it? I can read some people place it between the lens and GG, which apparently can affect focusing, while others use it outside the GG, photographer side. Which is best?
    Hi'ya, I have a linhof technika and, in this camera, the fresnel is *supposed* to be mounted behind the ground glass ie closest to you. The back has little clips there specifically to hold the fresnel and putting it in front of the GG would muck up the focus by shfting the position of the ground glass.

    On the other hand, the speed/crown graphics with a graflok back the fresnels goes in front of the ground glass and you probably muck up the focus by not having one in there.

    As far as I can see, you can always put a fresnel behind the GG but you can only put one in front if the specific camera is supposed to have it there? Someone please correct me if that's wrong.

  10. #10

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    Re: What is a fresnel lens?

    Andrew, a fresnel was optional with Graflok focusing panels for Pacemaker Graphics. The fresnel-ground glass sandwich (or ground glass without fresnel) sits on bosses on the casting. Castings intended to be used with a ground glass without fresnel have high bosses; those for the fresnel-ground glass sandwich have low ones.

    All Pacemaker focusing panels (of the same size) have the same casting number. If you don't have one as it came from the factory to compare another one with, it isn't easy to tell whether the 'nother one was delivered with or without a fresnel. Perhaps later today I'll measure a known "without" and a known "with" and report.

    With Pacemaker Graphics there's no camera specificity, there's focusing panel specificity. If you have one whose focusing panel has a ground glass without fresnel and want to add a fresnel, put it behind the GG (i.e., between the outside world and the GG).

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