Nathan, I have this lens and the CF III, and there's no impact on image circle. Given that the CF should be much larger than the front element of the lens, it's difficult to see how this could occur with the recommended filter. Can I ask what the front mount thread size is for your filter? The CF IIIA is not listed in my Schneider CF catalogue. One possible explanation is that the CF IIIA may have a smaller front thread size, capable of vignetting with this lens. The CF III has a front thread diameter of 86 mm.
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