In article <327AE77C.6583@uakron.edu>, birdsal@uakron.edu says...

>I recently had the chance to test both lenses side-by-side. I owned a >70-210/4-5.6 AF D but had heard that the f/4 constant was a better lens.
>When I had the chance to purchase a mint one at an attractive price, I
>did so, and checked out both lenses carefully, using Elite II 100 in a >tripod-mounted N6006. The lenses were very nearly equal in optical
>quality. The f/4 was marginally better wide-open at 70mm, but from
>105 on through 210, I noticed virtually no difference in contrast,
>center and edge resolution, or fall-off.
>The f/4 is faster at the long end, of course, but it is also bigger
>and heavier, and the AF is noticeably slower. The f/4-5.6 is shorter
>at 70mm but lengthens at longer focal lengths, and at 210mm is actually
>a bit longer than the f/4. I prefer the balance and general feel of the >f/4-5.6. Overall, I would say that the f/4 was very slightly better
>than the f/4-5.6, but not enough to justify (for me, at least) the
>additional size and weight and the slower focusing. I returned the f/4
>and have been happily using the f/4-5.6.

Given a good sample of the f4-5.6 version (I have seen noticeable
variation in these) and the f4 version (little variation), the above
is understandable - though if you are checking edges, and not corners,
you may be missing the greatest differences between these lenses, even
stopped down a bit (good f4 versions are good to the corners throughout,
whereas most 4-5.6's are not, even stopped down [especially at the short
end]). The AF f4 has an early barrel design (current AF barrels are
nicer) with the thin, awkward MF ring, making the f4-5.6 preferable
for MF use, though the "E Series" version of the f4 is better yet
(and has an almost all-metal barrel). At 210mm, f4 is a noticeable
advantage over f5.6 maximum aperture when hand-holding. Also, the f4
version is quite good on the TC14A by f5.6.
Hope This Helps