I recently bought a 75 mm Grandagon-N f/4.5 lens for my Toho FC-45X. It is advertised as having and image circle of diameter 195 mm. I also got Toho's eccentirc lensboard. Together with the normal movements, I can get pretty close to the limits of the image circle. So far I've only made one exposure---of a building facade---and I was pushing the shift capability. I don't notice any significant light falloff, but there does seem to be some loss of fine detail on one side and in one corner. I think this is most likely due to the front standard being slightly out of alignment, thus producing a slight swing. Some tests just viewing the gg image seem to confirm that conjecture. But I began to wonder how I would know that I was reaching the limits of what the lens can do. Presumably the size of the image circle is defined in terms of adequate definition, but what standard do they use?
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