But the light coming through the 8x10 iris is being spread over four times the area. If the intensity per unit area is the same with both lenses, the surely must mean four times as much light energy came through the 8x10 iris, even though it's no larger. Yes?
Yes but the lens is also collecting four times as much light from the subject.
"People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.