Thanks for your helpful comments Keith.
Yes up till now I've tried to avoided add extra glass for the reason you mention (but see below *). However recently I've seen some VERY impressive results using the Raynox - and it has the advantage of being relatively cheap (c.£45).
As said, I'm "only" looking to increase magnification to about 4:1 however at the same time I would ideally like to retain camera control over aperture. My one previous attempt at using a reversed lens set-up resulted in an unusably (in the field) short working distance and problems stopping down just prior to shutter release. Manually locking down the iris gave a very dark image and made critical focus quite difficult. Is there a technique for dealing with this problem?
* I have also tried reversing a Nikon EL 50mm F/2.8 (enlarger lens) onto the Nikon 105mm. This had the advantage of retaining aperture control but gave an uncomfortably close working distance for field work.....and of course it also means I was adding extra glass
.
Another option I'm considering is to add another set of Kenko extension tubes? This would give me a maximum 136mm of extension with no loss of image quality - excepting a worsening diffraction limitation effect of course. If this works I might also consider getting the Sigma 180mm macro lens to help increase working distance.
Any thoughts on this last suggestion?
Bruce