# Focal Length Question

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

1. ### bvySubscriber

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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 BMember

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

- Leigh

3. ### bvySubscriber

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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 BMember

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- Leigh

5. ### Helen BMember

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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. ### bvySubscriber

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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.)