Here is a link to various magnification ratio's. OK its a Nikon link but then knowing me it has to be.
http://www.nikonlinks.com/unklbil/macro_adapter.htm
Leif,
Lack of dof really starts to become an issue even with a reversed 24. Although adding a tc or tubes will increase magnification pic 2 of the spider set shows how much dof has been lost in the process. I think this is where sensor resolution becomes a serious asset as it allows for a fair crop of an image at lower magnification.
Too windy to play outdoors today but I am still searching for the best compromise. Wish I had the reach of a 105 in a macro lens now.
For backpacking and only using the D200 pop-up then a few sheets of kitchen roll and a bit of masking tape is probably as rugged and small as you'll get. As the dragonfly/fungii master with macro as your primary tool, then if I was in your position there would be only one route I would go, and that is a proper job.
Nikon SB-R1 Close Up Remote Kit or Sigma EM-140 DG Ring Light Flash.
The Nikon unit being the more versatile of the two, and you can add additional flash units at a later date. i-TTL so no flash knowledge or calculations required.
Don