In article <4j62km$493@inet.atc-usa.com>, rwilson@inet.atc-usa.com says...

>I've been eyeing a 105 F2.8 AF Micro-Nikkor for nature work
>(flowers, butterflies, etc.). However, it dawned on me that
>perhaps I could use my (rather ancient) 105 F2.5 lens with
>either extension tubes or close-up lenses (filters) for the
>same purpose.

I would not use that lens on tubes (performance of the AI-AIS
version, at least, is not great at closest focus at wide
apertures, let alone on tubes). Performance may be good at
smaller apertures with a tube. Borrow a tube and try it first,
if possible. Performance with the Nikkor 4T and Sigma 52mm
achromats can be excellent with many lenses at smaller apertures.

>I realize that the 105 2.5 is not a flat-field lens, but
>I don't think the 105 Micro is either.

The Nikkor macros are all flat-field, except the 55mm f3.5.
(Heck, most of the rest of Nikkors are flat-field, also....)

>Just how do you go about calculating the necessary amount
>of extension and any exposure compensation when using a
>lens with extension tubes? How is the focusing range
>affected? I know that I won't be able to focus to infinity.

Try it. If you have TTL metering, the exposure problem is
solved, also. If you don't have TTL ambient or flash exposure,
try a few 1/3 stop brackets around an exposure with the lens
opened 1/2 - 1 stop wider than usual (keep notes about exposures
and focused distances, and use slide film, which will reveal
exposure errors).
Hope This Helps