  The gravitational deflection of light is a critical test of modified theories
of gravity. A few years ago, Gibbons and Werner introduced a definition of the
deflection angle based on the Gauss-Bonnet theorem. In more recent years,
Arakida proposed a related idea for defining the deflection angle in
non-asymptotically flat spacetimes. We revisit this idea and use it to compute
the angular difference in the Kottler geometry and a non-asymptotically flat
solution in Horndeski gravity. Our analytic and numerical calculations show
that a triangular array of laser beams can be designed so that the proposed
definition of the deflection angle is sensitive to different sources of
curvature. Moreover, we find that near the photon sphere, the deflection angle
in the Horndeski solution is similar to its Schwarzschild counterpart, and we
confirm that the shadows seen by a static observer are identical.
