First rule of buying a supertele is buy the focal length that will allow you to shoot without TC’s most of the time.
Thanks, Gary. Words to consider seriously ...
For me, the 400mm will be an effective 600mm on a D500 ... which is all I really need + I have the added speed.
Still, a 600mm would give me and effective 900mm on a D500 ... which I do need as often as not
The trouble is, the 400mm FL ED is
much sharper @ f/4 than is the 600mm:
400mm f/2.8E FL ED Review
600mm f/4E FL ED Review(Note: I wish they would stick to the same graph format, so the values could be more easily compared.)
Regardless, F/4 is clearly the 400mm's sharpest aperture, while f/
8 is the sharpest for the 600mm ...
For a wildlife lens to be at its uttermost @ f/4 is
so important, and even exceptionally-sharp @ f/2.8, really not much else needs to be said.
Meanwhile, being its sharpest @ f/8 is what landscape lenses are known for ... but is not really what anyone wants in a super-telephoto lens.
I would be willing to bet that, with these performance values, the 400mm with a TC 1.4x III would be equivalent in sharpness @ f/4 (equivalent to 840mm f/4 with a 1.4 TC III + D500) as the 600mm would be @ an equivalent 900 f/4 bare on a D500.
In other words
where a lens is at its sharpest (f/4?, f/8?, or f/11?) is germane to
how well it takes a TC ...
That said, here's a shot I took yesterday, with my 300mm VR II f/2.8 and a 2x TC III.
Keep in mind, the 300 VR II apparently has an even worse curve than the 600mm f/4E FL ED, with its sharpest being @ f/11:
300mm f/2.8G VR II ReviewStill, even though f/2.8 is its
lowest sharpness point, at an equivalent 600mm f/5.6 (with a 2x TC III + D500) affixed to my 300mm f/2.8 VR II, the images are fairly sharp ...
Thus I think the
much sharper (wide-open) 400mm f/2.8E FL ED will do even better with a 2x TC III on it ... and simply exemplary with a 1.4 TC III on it