Further to previous message, here is the Seidensticker and Schmidt-Kaler map of how extinction varies over the face of the Coalsack. (some recent authors find slightly higher values.)
This is essentially the dimming of background light, at an observation wavelength approximating visual observation.
I might continue this cogitation about the Coalsack in the Science Forum!!
Note added in later edit:
Yes, bad galaxy man continues this discussion of extinction in the Coalsack, in the Science Forum!!!