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Interesting topic...
FSQ-106N,ED(EDXI&II) is a four element in two group design (two doublet Petzval), the Canon 500mm lens (wait for it) 17 elements in 13 groups! Serious amount of glass, but the quantity of glass has nothing do with this equation. Looking at the Canon 500mm lens at the business end i.e. where you attach the camera, max aperture is 52mm or typical filter size. This wouldn't be the image circle size. I would expect that to be less. Perhaps 48mm or so. This is similar to the FSQ-ED with either the optional Focal reducer or extender which is 44mm. However, natively (F/5), the FSQ has an image circle size of 88mm. Massive! If you were to put the lens on a large format camera, I'd expect to see considerable vignetting, though it would need to be something larger than the KAF-6803 chip that comes in at 38.6mm x 37.7mm. Technically the FOV would be illuminated but the drop of in light in the edges would be more severe. Light drop off on the FSQ with this chip is ~300 ADU - very little. Its difficult to get specifications on the colour correction for the Canon lenses. Given the high quantity lenses, I don't doubt one moment that they are world class, however it would be good to know. The new FSQ for example is corrected to 1000nm, well up into the IR range, designed to reduce star bloat. I'd also be very interested in how flat the field is across the image plane between the two.
At this stage, I think its difficult to draw a conclusion. I admire the wide field work lenses provide, but at fairly close focal length I'd say the FSQ has the edge. I wont be selling mine in a hurry (put it that way). Perhaps a more accurate test would be the Canon 400mm F2.8 and the FSQ-ED with reducer delivering 385mm F/3.6.
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