Subtraction

This is a bit of a puzzle (to me). On a dark and wet day, I experimented to see if putting a filter holder, that was a bit too small, on my new 40-150mm lens would cause vignetting. I thought if I took two exposures, with and without, and then subtracted one from the other in Photoshop, any differences would be revealed.

As I hoped, the result was almost black - no differences. I brightened the result by 5 stops and got this. Differences are the bright bits. The bright lines are because I must have moved the tripod very slightly when putting the filter holder on. The foreground (a door frame) is visible because the light must have changed slightly between exposures.

But what is the pattern over the whole image? It looks like a diffraction pattern, but where does it come from, and why is it revealed in the difference between two images?