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A fisheye is not a fisheye when used underwater.
EXPLANATION
The focal length of a lens is very much affected when immersed in water. The reason is that the angle of refraction is dependent on the relative difference between the indices of refraction of the incoming medium and the medium of the lens. To be precise, the formula for the focal length is: 1/flens = (nglass - nincident)*(1/R1 - 1/R2), where R1 and R2 are the radii of curvature of the entrance and exit of the lens. For a lens in air nincident is close to one. For water it is 1.33 or so. For various types of glass it?s often in the neighborhood of 1.5. You can experience the effect yourself if you try to read some, waterproof, printing under water. Your eyes? focal length change dramatically.
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