Jdef,
thanks for the reply. No, my 8x10 and 5x7 setups are identical, and checked, i.e., if focused on the same subject the infocus parts are equally sharp when shot at the same fstop with the same lens, as viewed through a 4 power magnifier. I maintain that the only difference is that I'm shooting at a smaller fstop with 8x10 and the loss of resolution is caused by diffraction.
Take care,
Tom