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Help with my large mirror and camera.

  1. Aug 19, 2009 #1
    This isn't homework, I really have this setup. :-)

    I have an 18" mirror with an 80" focal length. The light from a point light source at twice that distance (and offset to the side slightly) bounces it's beams off the mirror and back to form a perfect image of itself next to the light if you hold up some paper.

    Moving the paper back the image expands, and keeps expanding of course.

    I want a lens that can sit behind that convergance point and make the light paralell so it projects the same size image no matter how far you move back from it.

    This is to try and overcome issues with cameras where the diverging light is blocked inside the lens by the iris and only the center of the mirror is lit up instead of the entire thing.

    Some of you might recognise this as a very similar setup to a Focolaut test.

    So the question is... what kind of lens do I need? I have a number of random biconvex lenses I tried, but none have the right proprties to make the light paralell. Several work well to focus the light into a nice sharp image, but cameras can't seem to focus on that.

    18" diamater mirror
    Parabolic shape
    80.6" focal length

    If anyone knows what properties a lens would need, does anyone know where I could get one? If it's not something "standard" I can find in a catalog somewhere that is.


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  2. jcsd
  3. Aug 20, 2009 #2

    Andy Resnick

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    My first thought was that you need a telecentric lens, but reading more closely, you are asking for (essentially) an infinite depth of field (...'it projects the same size image no matter how far you move back from it')- you are asking to porject images of the source. I don't know a simple lens for that.

    If, however, you are asking for a condenser lens- a lens to collimate light from both the source directly and the image of the source created by the mirror, that's a little simpler

    Can you be a little more specific? If it's the latter, a simple plano-convex lens placed 1 focal length away from the (two) sources will generally suffice.
  4. Aug 20, 2009 #3
    I'm not really sure what I am asking for, I have been trying so many variatons of cameras and lenses and setups. :-)

    Here are some diagrams showing the problem with trying to photograph the reflected light.

    SchlierenDiagram.png CameraCloseup.png

    So I need to stop the light from spreading out so fast. Something that would make it slowly converge instead might work.

    With a single convex lens I can make a very nice projection onto an index card, or the bare sensor of a CCD. But most digital sensors are TINY and I can't get them close enough to even try and focus it onto, even if I can successfully rip out the lenses in front of it.

    So I need to work with the camera glass elements intact.
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