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Effect of distance in collimated space. NOT for homework.

  1. Jul 6, 2012 #1
    Hi All,
    I apologize in advance for the elementary nature of my question, but I have made a genuine effort to understand what is going on using a couple of optics textbooks and remain confused. My background is not in optics but I definitely am just missing something basic.

    I have an optical set up I posted here: http://i.imgur.com/XUH0w.png . I am using a cell phone as my imaging device, shown on the far left of the diagram. It has a lens with focal length of 4.6 mm and I have set it to focus on infinity, so the lens and CMOS should be separated by a distance of 4.6 mm. I understand the system for the most part except for the distance I have labeled d4 and similarly the distance I have labeled d1. d1 I basically make as small as possible -- I have an aspheric lens pressed right about against my cell phone and I'm not planning on changing that distance. However, I'm not understanding the effect of measurement d4 on the system's overall magnification. The linear magnification I get from the combination of the first two lenses, or at least what I thought it was, was just (focal length lens 3)/(focal length lens 4), or about 8. However, I notice in my system that the distance d4 changes the image I capture. If I have distance d4 very small, I can fill my cmos sensor with the image of my intended object (my object is actually an image coming out of a fiber bundle).

    Anyway, can someone help me to understand the effect on the systems magnification of the distance in collimated space between my "objective" (aspheric lens) and "tube lens" (30 mm achromatic lens). Thanks and again apologies for asking such a rudimentary question.
  2. jcsd
  3. Jul 6, 2012 #2


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    I would not expect any influence from d4. The light is parallel and nothing changes with distance, neglecting possible nonlinear effects in your setup.
  4. Jul 6, 2012 #3
    Yeah that's what I would expect too. It's causing quite a big difference, so I must be doing something wrong in setting it up. Thanks
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