Nikonian72 wrote:
This statement makes no sense...
It actually does make sense (except, yes, it's 1:2, not 2:1).
Tamron has been making SP 90mm macro lenses for more than 30 years and there have been a number of different version... several manual focus and several autofocus. The Tamron SP 90mm is a "cult classic" macro lens.
Several versions of earlier, manual focus Tamron 90mm SP macro Adaptall lenses were 1:2 on their own and came with a matched 1:1 adapter/teleconverter that looks like a standard teleconverter, but actually isn't. The 1:1 adapter fits between the interchangeable Adaptall mount and the lens body, so is only usable with this or other Adaptall-series lenses. In case you are unaware, Tamron's Adaptall system allows easy swap of the bayonet mount on lenses (all manual focus and mechanical aperture control), so that they can be used on a very wide variety of different manufacturers' cameras.
I have one of these lenses, along with Adaptall mounts to be able to use it on Canon EOS/EF, Canon FD, Nikon F, Konica K/AR, Pentax P/K and Olympus OM cameras. I also have the matching 1:1 adapter, but never use it. Instead I use standard macro extension tubes to increase the magnification of the lens, since those tubes are usable with all my other lenses in that mount, too.
Here's my Tamron SP 90mm with an EOS Adaptall mount installed and a Nikon Adaptall alongside (left) and the lens mounted on one of my Canon DSLRs (right)...
Here's a shot taken with that lens (with 20mm extension tube, if memory serves)...
The Tamron SP 90mm Macro manual focus/Adaptall came in a couple f2.5 versions initially, then in one or two f2.8 versions.
Current and recent autofocus Tamron SP 90mm have all been capable of 1:1 without any adapter. AFAIK, all the autofocus versions have been f2.8 lenses.
The latest and greatest of the modern autofocus versions is the Tamron SP 90mm "VC" and "USD"... stabilized and faster/quieter ultrasonic focus drive... it's also now an "IF" or "internal focusing" lens.
They still offer the SP 90mm non-VC, non-USD, non-IF version as a lower cost alternative.
None of this addresses the OP's questions about using a 1.4X on that 40mm Nikkor.
Don't have that lens, so I don't really know. Nikkor teleconverters.... probably not. (I know Nikon has a teleconverter compatibility list online, which you can check.)
Third party TCs... maybe. I'd look at the Kenko, in particular. They have two or three series, at different price points: "Pro 300" are their higher quality, "MC-4" are lower cost... and I think a new "HD", don't know how those compare.
However I would recommend trying a 2X, instead of 1.4X. Combined with a 2X you'll have a more useful effective 80mm f5.6.... Personally I don't think 1.4X to make a 56mm f4 is worth the trouble, not enough "longer" than the 40mm itself.