I've swept away the dust from my old optics books. Is this calculation correct?
If the radius of curvature of a lens is 25 mm and the index is 1,67 the lens power is:
F=(n'-1)/r
F=0,67/0,025 = +26,8 D
Then for this lens the focal distance is:
F = n' / f '
26,8 = 1,67 / f'
f' = 16,04 meters.
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