  General relativity predicts that gravitational lensing near black holes will
produce narrow "photon rings" on images. Building on recent work of Johnson,
Lupsasca et al. focusing on circular rings, I calculate the long-baseline
interferometric signature of a narrow feature of arbitrary shape and intensity.
The shape information is contained in the oscillation frequencies of the
complex visibility as a function of baseline length, for each baseline angle.
These results raise the possibility of measuring the detailed shape of a photon
ring and comparing to the precise predictions of general relativity.
