We study the farthest-point distance function, which measures the distance from
z\,\in \,\mathbb{C} to the farthest point or points of a given compact set
E in the plane.
The logarithm of this distance is subharmonic as a function of
z, and equals the logarithmic potential of a unique probability measure with unbounded support. This measure
{{\sigma }_{E}} has many interesting properties that reflect the topology and geometry of the compact set
E. We prove
{{\sigma }_{E}}(E)\,\le \,\frac{1}{2} for polygons inscribed in a circle, with equality if and only if
E is a regular
n-gon for some odd
n. Also we show
{{\sigma }_{E}}(E)\,=\,\frac{1}{2} for smooth convex sets of constant width. We conjecture
{{\sigma }_{E}}(E)\,\le \,\frac{1}{2} for all
E.