Newton's lens formula
= Newton's lens formula
{c}
{title2=$u'v'=f^2$}
For a converging <thin lens> forming a <real image> of a real object, let $u,v$ be positive distances from the lens and $f$ the <focal length>. Set $u'=u-f$ and $v'=v-f$, measuring from the neighboring <focal points>. Substituting into $1/u+1/v=1/f$ gives $(f+u')(f+v')=f(2f+u'+v')$, so $u'v'=f^2$. Other object-image geometries require an explicitly consistent signed-distance convention.