"I know of the respective hint from Schneider. But I understood it as that the filter as such is usable wide open already (where itself already needs factor-3 exposure compensation) and that the advised closing of aperture by 2 stops is meant to raise the usable image circle up to the corners. Where image quality otherwise did not matte mucgh as being lost in vignetting anyway. But I admit that I never checked this at lens performance tables. And I am aware that stopping down has effect on the gradation of shades in front of the lens. So that this effect already may have taken into account for the advised stopping down at the design of the filter. And thus the filter as such not being at optimum wide open."
AGX,
All you say is indeed correct in my book. When I have time I will run some definitive tests with the 65mm F8 Super Angulon Center Filter. According to what I have in my literature it really only covers 9x12cm, which is perfectly fine for my uses. I buy 9x12cm film from the usual European shoppes, and I have a lot of Linhof and Norma 9x12 film holders. But my USA made Omega DII enlargers really key to 4x5 sizes, and I have 4x5 film and 4x5 holders in quantity in this size. The Center Filter is like the cream in the coffee; I'll wager it improves the 65mm F8 Super Angulon, but we will see pretty soon now. I put a 4" Wratten #12 Gelatin Kodak Filter behind the rear cell; So I will get the combo of the Yellow K2 filter inside the body, and the correct center filter on the front. So multiple exposure compensation is required; 1.5 stops for the center filter density, 1 stop the #12 Wratten Gel filter density. So 2.5 stops total exposure factor. And then, two stops down to avoid what I think is mechanical vignetting from the edges of the center filter. This is all a guess so testing will tell the tale. Much fun ahead. Will need to bring along a tripod. The early silver Leitz Tiltall is my preference with the Norma Handy setup. Mighty Handy Indeed! I have three Graphmatic 4x5 backs loaded with HP5+, and another three Graphmatics loaded with 4x5 TMAX400. For handheld shooting I can push process the film up to EI 1600 and still get reasonable quality. Again we will see. My wedding flash setup should do the job coverage wise with the Handy; again we will see