In article <322b99ed.8830369@news.buffnet.net>, bgarman@buffnet.net says...

>I am the owner of an N90s and would like to get a lense for long range
>nature that my 400mm cant reach, is there such a lense out there that
>would give me good reach with fair performance without a heavy price
>tag. What about a doubler for my tamron 200-400, is worth a try.

Since by all reports the Tamron 200-400mm isn't great at 400mm, a 2X
would probably provide a soft, very slow, and useless combination.
The best buy for Nikon owners in sharp longer lenses is the older
(larger, non-macro) style 500mm f8 Nikkor mirror, though it has
most of the drawbacks (for some people) common to mirror lenses
(this mirror makes a sharp 700mm when used with the Nikkor TC14/14B
converter, also). When you use lenses 500mm and longer, the air
quality, light levels, film speed, and subject movement speed become
critical for success, and the very long lenses are VERY hard to use
for making sharp, contrasty images - many conditions need to be
just right for good results. (BTW, I am considering selling a
Russian MTO 1000mm f10 mirror in nice shape for a reasonable
price [it fits any camera, since it has an interchangeable "T"
mount - it is not razor sharp, but contrast and sharpness/illumination
evenness to the frame corners are good, and it gets images shorter
lenses can't.)
Hope This Helps