The field of view crop for the Nikon DX bodies is 1.5x, so you just multiply the mm. Your 120-300mm on a DX body will "look like" a 180-450mm without the TC. With a TC, do that multiplication, too.
The big catch here is probably determining which TC you want to use. The 1.4x will give better image quality and eat only one stop of light but does not "reach" as far as the 2x. However, if you have the new "III" version of Nikon's 2x TC, this might be a good option even though you lose two stops of light.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA
In the circumstances you describe, for me the much more effective AF system of the D800 (compared with that of the D3200) would win out over any perceived difference in "reach" between the twon cameras.
The 120-300mm you referred to is presumably the f2.8 Sigma?
A 1.4 TC will lose 1 stop, giving a 420mm f4 combo A 2.0 TC will lose 2 stops, giving a 600mm f5.6 combo Stacking these TCs will lose 3 stops (8 times less light entering the camera) giving a 840mm F8 combo in theory. In practice it is probably worse because forcing the light to pass through all that extra glass will cause some extra attenuation. Stacking 2 TCs is likely to cause so much image degradation that I would not hold out much hope of getting a sharp image. At very long focal lengths, vibration becomes a bigger issue too. Reckon you would be better off using the high MP count of D800 and cropping.
Now the 28-300mm. this lens is f5.6 wide open at the long end. Adding 1.4 TC will lose 1 stop = 420mm f8. the D800 and D3s will still AF but not with all sensors so tracking ability is probably lost if shooting BIF. Adding 2.0 TC will lose 2 stops = 600mm f11. Auto focus capability is lost and manual focus compromised by dim viewfinder image. Stacking the TCs = f16 = no AF and viewfinder image so dark that manual focus virtually impossible.
The 28-300 lens is not as sharp as the 120-300 at 300mm. Not even nearly in the same league, so the image quality losses with added TC or TCs will be worse. Added to focussing difficulties and lower shutter speeds/higher ISO forced by the small aperture, I would not rate my chances of getting even a single keeper with this combo. Try it at home before embarking on your trip
>I forgot to ask will the Nikon 2x version II work with the >Nikon 28-300?
No! The Nikon TC's will not even mount on the 28-300mm.
>Or can it be made to work.
Personally I wouldn't bother. The Kenko Pro 300 series TC's will mount though you will likely not be happy with the AF or the optical performance. There might be an interference problem with the rear element of the lens and the front element of the TC that I would check before even considering mounting a TC to that lens. Good Luck and Enjoy your Nikons!
The best would be D3200 (technically speaking) with 120-300mm and 1.4x Sigma 2x on 120-300mm is really bad unless you are at F11 and even then not so good. For this kind of situation I have Sigma 50-500mm OS stopped to F8 (one stop) and it is excellent. Sigma 120-300mm F2.8 OS is the best lens out there and it works well with 1.4x Sigma TC but that is all you can do.