= Focus
{disambiguate=optics}
{title2=$F$}
{wiki}
= Optical focus
{synonym}
An <optical focus> is a point where a specified bundle of <light rays> converges, or where its backward extensions meet. For a converging <thin lens> receiving <light rays> parallel to the <optical axis>, the rear <optical focus> is $(f,0)$ when the <thin lens> is at $x=0$; its front <optical focus> is $(-f,0)$. An object's image is generally elsewhere: the <optical focus> describes the parallel incoming bundle, while the <thin lens> equation locates the image for a finite object distance.
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