Quote Originally Posted by couldabin View Post
But the light coming through the 8x10 iris is being spread over four times the area. ... Yes?
No

The light coming through the 8x10 iris is coming from a subject that is four times the area, and being spread over film that is four times the area, so it is a wash.

It would only change if you changed the magnification to fill the 8x10 film with the same subject as previously filled the 4x5 centre (and then only if you were working at or near macro magnifications).