Well, for the first time I've totally caved to popular demand and done a test I had little interest in doing. But after I did a Quick-Take post on the new Nikon 80-400 AF-S VR lens I received about two-dozen emails and comments asking if the 70-200 f/2.8 AF-S VR II lens with a Nikon 2X III teleconverter was as good as, or better than, the new 800-400 AF-S VR.
My first impulse was to do Standard Internet Response #1 -- give an absolute answer, such as 'obviously not', despite having no facts to back that answer up. Then I considered Standard Internet Response #3 -- give a useless, but factual, answer like, 'well, if you have a 70-200 and teleconverter already, that's certainly adequate'. (I never use Standard Internet Response #2 - the 'if you're a good enough photographer it doesn't matter which you use' response, nor S.I.R. #4 -- 'Google is your friend'.)
But, since it really is a reasonable question and a lot of people seemed interested, we set up to Imatest the 70-200 f/2.8 VR II / 2X III combination. Please be aware that our longest testing distance is 40 feet, which isn't ideal for testing 400mm lenses, but it's the longest we have. (I'm pretty comfortable it's a longer testing distance than anyone else has, too, except maybe DxO and they aren't really sharing information about their testing set up). Results may be quite different at 300 feet. I'm not sure which way they'd be different. The 70-200 seems sharper at this distance than it does at infinity, at least that's what most people say. On the other hand, teleconverters are generally tuned for long distance shooting. So I just don't know. (BTW - "I don't know" is not a listed S. I. R.)
We used an identical setup to the tests we ran last week on the 80-400 AF-S and 80-400 AF lenses to test the 70-200 f/2.8 with 2X combination. The MTF50 results are shown in the table below. The bottom line, from a resolution standpoint, the new 80-400 is clearly better. The previous 80-400 is better than the 70-200 with 2X right in the center, but outside the center the 70-200 with TC is very close.
Center MTF50 Avg MTF50 Avg. Corner MTF50
Nikon 80-400 AF-S 820 675 480
Nikon 80-400 AF 725 575 410
Nikon 70-200 f/2.8 with 2X 600 560 440
What does it mean? Mostly it means if you're shooting at 40 feet distance the 70-200 VR II and 2x teleconverter will get you a nice usable image, but not as good as you would get with the 80-400 VR II.
The old 80-400 AF lens is better in the center than the 70-200 VR II combination, although that's just right at the center. Less than 1/3 of the distance away from the center, the two are even.
I can't say the results would be the same if the shooting distance was near infinity, and I'm not sure how they'd change. The 70-200 alone is reputed to be a bit less sharp at infinity, though. On the other hand, the teleconverter might well have less of an effect at the longer shooting distance.