= Focal length
{title2=$f$}
{wiki}
The <focal length> $f$ specifies the signed distance from an ideal <thin lens> to its <optical focus> for incident <light rays> parallel to the <optical axis>. A converging <thin lens> has $f>0$, with a rear <optical focus> beyond the <thin lens>; a diverging <thin lens> has $f<0$, with a virtual <optical focus> on the incoming side. The <focal length> is a property of the <optical lens> in its specified surroundings, not the image distance for every object. For example, a converging <thin lens> with $f=1\,\mathrm m$ images an object $3\,\mathrm m$ away at $1.5\,\mathrm m$, as computed below. https://www.feynmanlectures.caltech.edu/I_27.html[Feynman, Leighton and Sands, The Feynman Lectures on Physics I, §§27-3–27-4]{ref}.
Back to article page
