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\title{Complete Optics: Lens Equation}
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\section{Lens Equation}

For a thin lens with focal length $f$, object distance $s$, and image distance
$s'$, the paraxial relation is

$$
\frac{1}{f}=\frac{1}{s}+\frac{1}{s'}.
$$

It is the workhorse formula for elementary imaging systems.

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