Can someone please help stupid me understand this. I've heard it said that in terms of brightness, f/4 at a given focal length is the same whether on an SLR, MF, or LF. However, I'm not quite sure I understand that.

I understand that the same amount of light is collected on a given aperture size. However, for an f/4 lens on an SLR, that light is being put on a 35mm frame. For a LF, it is being put on a 4x5 or 8x10 piece of film. It would seem to me that in terms of "brightness", it certainly isn't the same, even at a given f-number.

Can someone please explain this to me.