Exactly. The thing that really matters for light gathering is Etendue -- the product of the area of the source and the solid angle that the system's entrance pu

Exactly. The thing that really matters for light gathering is Etendue -- the product of the area of the source and the solid angle that the system's entrance pupil subtends as seen from the source -- which is conserved (in a lossless optical system). [1] https://en.wikipedia.org/wiki/Etendue https://en.wikipedia.org/wiki/Etendue