 Solution

A photon follows a null geodesic, whose spatial path approaches straight lines far from the deflecting mass. Extend the incoming asymptote forward: this is the reference path the photon would follow if the mass were removed while its incoming direction and transverse offset were held fixed. The impact parameter $b$ is the perpendicular distance from the mass to that incoming asymptote. It is not, in general, the distance of closest approach.

The positive deflection angle $\delta$ is the angle between the outgoing propagation direction and the continued incoming direction. The photon bends toward the mass, so a ray passing above it has a downward outgoing slope in the figure. In the weak-field approximation for an isolated spherical mass, Schwarzschild light deflection gives $\delta \simeq 4GM/(b{c}^2)$ for distant endpoints. This angle differs from the apparent image angle measured from the observer's line of sight to the mass.