This is offered for comparison to the next. This is not cropped (just resized for posting) shot at ~10 feet with the 600mm Sigma mirror (handheld but leaning on the windowframe). The next is cropped from a shot with the 70-200 f/4L. Both were taken from (and through) the same window glass so there should be some basis for comparison as to whether a good lens cropped or a bad lens full frame is better. There is one other thing not tried: a good lens with an extender. If anyone would like to donate a 300mm f/4L IS for testing, I'll buy the extender. I shot several versions of this but only a flat side view had enough depth of field to use. This one is a bit soft on the back of the head; I have another sharper there and a bit soft on the bill. Rather few people use 600mm lenses at this distance for good reason.

What looks like doubling on the beak is a groove on the edge and not camera motion.