3d – Perspective of a Point Light

I’m trying to calculate shadows by projecting the silhouette of each object seen by the light source.
(The shadows are just going to go on planes.)
To get each silhouette I take a snapshot from the point of view of the light source looking at the object.

But I’m confused what the field of view of the snapshots is analogous to in the real world.
I was thinking perhaps the size of the light source?
But there’s both horizontal and vertical field of view, snapshots in all directions, and I’m lost.
So I’m asking what dictates the perspective of these snapshots.