the SLR body will vignette (clip the light) if the film holder is to close to the body of the SLR. On another point using a 50mm len to compose wont work to well (if at all) as the focus point of the len is 50mm, I think your filmholder is going to be about 100-150mm out from the back of the SLR body so you get a full 4x5 image.