OK this is the part Ive never fully understood, if there is no fresnel the ground glass is on the same plane as the film when a holder is inserted, now the part I do never have understood, does placing a fresnel between the lens and GG shift the focus point due to moving the GG because you have put the fresnel between the GG and the wood mounting frame physically moving the GG away from the lens the thickness of the fresnel or is it a optical thing changing the focus point of the light because it passes through the fresnel before hitting the GG, and the result is that the GG is at a different plane than the film.