The new 400mm DO compared to 300/2.8 and 400/5.6 for sharpness

[Kayaker72]

That makes sense. To me, the difference in the two lenses gets down to if you prefer 300 f/2.8 or 800 f/8. The other two focal lengths (400-420 f/4 and 560-600 f/5.6) essentially overlap. I suspect, and the lensrentals data supports, that the 400 f/4 is optically a bit better at 400 vs 420 for the 300 f/2.8 + 1.4TC. I also suspect the same will be true at 560 mm / 600 mm in that the 400 DO II + 1.4 TC will be slightly optically better than the 300 f/2.8 II + 2xTC. Finally, I wonder about AF speed with the teleconverters. In the end, 300 f/2.8 is going to be a heckuva argument for most buyers.

Plus, it will depend on how it fits into your kit.

[neuroanatomist]

To me, the difference in the two lenses gets down to if you prefer 300 f/2.8 or 800 f/8.

Agreed. For me, since I already have 840mm f/5.6, 300mm f/2.8 is more appealing.