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Dustin Abbotts review, which I read is a touch overly enthusiastic, and unrealistic. From many images I have seen, roger's review, my personal experience with several tarmon lenses, although not this one, my deductions are,
The lens wil be good at 600 and f8. In good light, the lens does well esp for non moving objects. the modern camera high iso has certainly improved greatly, esp on full frame, where very good results can be seen at iso 1600, even 3200 with good processing. it appears that in 150 to 400 range, it will be reasonably close to 100-400L which is quite impressive.
Non of the consumer zoom lenses that go to 400 and beyond are particularly fast focusing. that includes 100-400L, which I consider marginal on my 5d3, for BIF. In fact, which I was shooting hummingbirds, I got more than twice as many in focus images with my 70-300L.
I think this lens may be the new perfect safari lens. For birders, shooting non flying birds, on a tripod, at say 1/300-1/500 sec on a tripod/ monopod, at f8, this will be a good lens also. BIF, I find focus speed is a critical factor.
I would also be interested in focus accuracy. my experience with tamron 24-70 vc has been very disappointing. and this is not something that reikan focal necessariy detects. I test the lens with reikan, and the lens does well on repeated refocus on the same subject. but not so much in the field, on its first focus attempt. I very much doubt the Tamron has solved that riddle. Focus issues will remain the Achilles heel of third party lenses, the precise reason canon is not licensing its focus algorithm to them.