Don't put too much emphasis on test results such as these, even if the tests are accurate, there is no point in buying the wrong lens because it is sharper. For example the Otus 85 might be even sharper, but you wouldn't buy it for your purposes. If you mostly need 600mm, the 600 FL is surely a better option than 400 + TCs.
400 is a lot closer to where I want to be than 85
Remember, most of these tests are done using a single sample of each lens, maybe they tested a good example of the 400 FL against a bad example of the 600 FL (at this level, QC is very high and I expect sample variation to be minor, but it is still possible one lens has been knocked and put slightly out of alignment). Other test sites could easily give different results. The only place which tests multiple copies of lenses is LensRentals, and they can't test lenses this long.
True.
Another consideration is
competence at Imatesting. At least one source seems to be in left field compared to other values given at other sources ...
One thing for sure is, they're
all super lenses capable of delivering wonderful results.
The Nikon MTF charts are outstanding for both lenses, they show the 600 FL is a bit sharper over most of the image but dropping off towards the corners, while the 400 FL is a tad less good but more consistent to the edges. For bird photography I wouldn't be concerned about edge sharpness since the images are usually part of the background and not in focus anyway. So going by that I would conclude the 600 FL is sharper! Going by the MTF charts, the 500FL is even better, see for yourself here: http://imaging.nikon.com/lineup/lens/index.htm
Interesting. I also agree edge-to-edge sharpness isn't that big a deal when you're framing animals, as everything in the background/foreground is going to be a blur anyway.
In the end, I would give more credence to Nikon's own testing than that of anyone else.
Nikon is the entity with the engineers,
and the resources, to
design and make these lenses ... which carries far more weight as to 'levels of competence' than a blogger who bought an Imatest
These sites usually test sharpness at only one distance, and as we know, some lenses are better at far distances and others better at close range. Do the tests above correspond to the distances you will usually shoot at? Impossible to say.
True.
I understand that you want to do your homework before making a big purchase, and it is useful to check these review sites but they can only say so much and you need to take their results with a grain of salt. It might be more useful for you to rent both lenses and get some hands-on experience with them. It might turn out that other factors might sway your decision one way or another.
Good advice.
One of the elements that sways me in favor of the 400 f/2.8E is the fact that most (who have shot both) rate the 400 is sharper, and stellar wide-open, placing it at a level above other FL ED lenses.
Still, everyone with experience seems to favor the 600 f/4 (which would be ~900mm bare on my D500).
They're both about the same price (only ~1,000 difference), and about the same weight, so the money/weight isn't really the issue.
I tend to favor lenses that are fast wide-open ... but I do want the reach
