  We present a method to compute exact expressions for optical observables for
static spherically symmetric spacetimes in the framework of the bilocal
geodesic operator formalism. The expressions are obtained by solving the linear
geodesic deviation equations for null geodesics, using the spacetime symmetries
and the associated conserved quantities. We solve the equations in two
different ways: by varying the geodesics with respect to their initial data and
by directly integrating the equation for the geodesic deviation. The results
are very general and can be applied to a variety of spacetime models and
configurations of the emitter and the observer. We illustrate some of the
aspects with an example of Schwarzschild spacetime, focusing on the behaviour
of the angular diameter distance, the parallax distance, and the distance slip
between the observer and the emitter outside the photon sphere.
