Bobby,
Although the 70-180 is a great lens, I would not use on it anything else
than the diopter you already have. Zooms are very tricky with extension
tubes, moreover if there's internal focus involved (don't know about the
70-180. Additionally, in this specific case you'd have to deal with an
even shorter working distance.
As John Shaw himself states on his excellent book, mounting reversed
lenses is not that simple; it's more a test-and-see issue than something
you can foresee. Luckily you don't have to buy expensive materials to
test it - I just glued 2 Cokin filter holders to get my male-to-male 52
thread adapter.
I found that a 50 mm 1.8 added to my 105 micro is of no use at all due
to vignetting. I'm going to test that same 50, reversed on my 200 f/4
AIS micro by the following days and post the results.
I would suggest using a prime lens (ideally a 105 Micro) with extension
tubes, or bellows, to go beyond lifesize and to 2X. The 200 focal length
is too long for that purpose, as is 400 mm. From 2X to 8X or more the
best option seems to be reversed 50 mm on bellows or reversed wide
angles.
Hope this helped.
--
Dr. Roger Eritja ([EMAIL PROTECTED])
Biologist
Mosquito Control Service - Consell Comarcal del Baix Llobregat
SPAIN
Ph. +34-936401399 FAX +34-936300142
