In article , dga2@po.CWRU.edu says...
>Has anyone seen or done a comparison of the Tokina lens with the Sigma
>Macro 400mm?? (......) Any other recommendations on 400mm lenses, >500mm lenses??

One sample I tried of the Tokina 400mm f5.6 (early, not special glass
version) was, to say the least, not worth owning. (Later versions, if
they are improved designs or better samples, may be acceptable.) I
do not know what is so difficult about producing a good (slow) f5.6
400mm (two Sigma APO's [not the macro version] were, to be charitable,
decent, and an antique Spiratone 400mm f6.9 was about the same quality) - there are inexpensive 300mm f4.5's around that are good. The Nikkor 400mm f5.6 is a really excellent lens, but it is over priced
(especially in view of its price only a couple of years ago, and its
price back then relative to other lenses in the line). If you can stand the slow speed, the Cosina 100-500mm f5.6-8 (actually slower), and the
older style Nikkor 500mm f8 (actually slower) may be worth looking for
(they are both sharp, but I would dare add only a TC14/14B to the
Nikkor - we're getting iffy, here, with speed.....).
Hope this helps.