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 Incoming links: Newton's lens formula

For the real object and real image, take positive distances

u = f + u^{'}

and

v = f + v^{'}

uv = f (u + v) ⟹ (f + u^{'}) (f + v^{'}) = f (2 f + u^{'} + v^{'}) ⟹ u^{'} v^{'} = f^{2} .

u^{'}, v^{'}

refer to the adjacent focal points, not the lens plane.