Focal Length Question

Discussion in 'Camera Building, Repairs & Modification' started by bvy, Jun 11, 2012.

  1. bvy

    bvy Subscriber

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    What would be the focal length of a hemispherical (half ball) lens of radius r? I'm only finding details related to ball lenses and plano-convex lenses. Thanks.
     
  2. Leigh B

    Leigh B Member

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    A hemispherical lens is just a plano-convex lens of appropriate dimensions (disregarding distortion & aberrations).

    - Leigh
     
  3. bvy

    bvy Subscriber

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    I see. The lensmaker's equation with radius of curvature of zero (?) for the flat side and, and r for the hemispherical side. I just need to find the refractive index of clear acrylic.
     
  4. Leigh B

    Leigh B Member

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    Google it.

    - Leigh
     
  5. Helen B

    Helen B Member

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    The radius of a flat surface is infinite, not zero (so the reciprocal of the radius is zero). You may be better off using Gullstrand's equation for a thick lens rather than the lensmaker's equation, which is only intended for thin lenses.

    Best,
    Helen
     
  6. bvy

    bvy Subscriber

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    Thanks Helen. I knew there were a few equations, and I think that's the one I need. (And, yes, I was actually referring to the reciprocal term in the lensmaker's equation. I misstated that.)