  An optical equation for null strings is derived. The equation is similar to
Sachs' optical equations for null geodesic congruences. The string optical
equation is given in terms of a single complex scalar function $Z$, which is a
combination of spin coefficients at the string trajectory. Real and imaginary
parts of $Z$ determine expansion and rotation of strings. Trajectories of
strings can be represented by diagrams in a complex $Z$-plane. Such diagrams
allow one to draw some universal features of null strings in different
backgrounds. For example, in asymptotically flat space-times $Z$ vanishes as
future null infinity is approached, that is, gradually shapes of strings are
"freezing out". Outgoing gravitational radiation and flows of matter leave
ripples on the strings. These effects are encoded in subleading terms of $Z$.
String diagrams are demonstrated for rotating and expanding strings in a flat
space-time and in cosmological models.
