I have a Rodenstock Grandagon-N 90mm F6.8 lens and do not have a center filter. I have noticed some light fall off with this lens and 4x5 film. I have not exper iemented in a controlled manner at various aperatures, but, was wondering. Are t here aperatures in which the light fall off will be the least? Are there aperatu res where it is the worst? I generally use this lens at F22-45 and the fall off is minimal. Just wondering if I can optimise the perfomance without a centering filter.