Basically how badly using a step-down ring will cause vignetting if the lens is stopped down as well? Specifically, I have got a Mamiya-Sekor C 80mm f/1.9 lens, which has 67mm filter ring, and I would like to try IR photography in medium format. The problem is that my IR filter is 52mm, and as Heliopan filters aren't exactly cheap, I'd prefer to use a step down ring instead if I can get away with it. Now obviously it would vignet a lot if you used it at full aperture, but how about stopping it down to say f/5.6? If I have understood correctly, that would basically mean that only the centre third of the lens would be used, so would this reduce the vignetting to manageable levels?