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Here is a 100% crop from a shot w the 600 f4 II + 2XIII at 1/800, f 8, ISO 1600 w the 1DX. I find the IQ more than just adequate (I processed w sharpness left at 3) and maybe better than I expected from the ISO chart of this lens. Of course, this is from the center of the frame where the resolution of the lens is highest. AF with the firmware update on the 1DX and this lens combo is quite good too - helps a lot to limit the focus range based on anticipated distance to subjects though - in this case, ca. 4.5-16m.

I'm now happier w the 600 choice since the performance with the 1DX + 2XIII seems very good. Being able to access 1200mm w AF and useable IQ is worth the extra pounds versus the 500mm for me.