I'm crossposting this from a Rolleicord thread. I have a Rolleiflex with a 7.5cm lens in it, and a Fresnel screen cut to fit from a Mamiya RB screen. My local repair guy says this won't work, because the Fresnel lens acts on the optical system, causing a focus shift. Apparently the Mamiya RB series are designed with an 80mm focal length in mind, where the Rolleiflex lens is a 75mm, and this is supposedly enough to throw off focus at one end or the other. This seems wrong to me; how would (say) a 65mm lens focus properly on an RB then? Or is are the retrofocal/telephoto lenses calculated to avoid this effect somehow?