Thank you all for your advice and comments, you've certainly made me realise that it would be a mistake to proceed without fully understanding exactly what I'm gonna achieve - the disadvantages as well as the advantages.
Graham and Keith - Thanks for pointing out the inverse relationship between extension tubes, lens focal length and magnification - I'm going to have to think carefully about that - I could have made quite an expensive mistake there (gulp!).
Graham -Thanks for the link I'll see what I can make of it.
Keith - I currently get a tad under 2:1 magnification using 68mm of extension tubes with my 105mm macro lens (see test shot of ruler below) so if 136mm of extension will only give me 2.3:1 then it's not going to be worth the expense and operational disadvantages.
Adam - Thanks for posting your first class DCR-250 results. Actually I generally use aperture priority when ambient light is low and I can get away with flash TTL. However I always use full manual exposure (I set both aperture and shutter speed) with manual flash control when I'm at >1:1 so I'm certain that the DCR-250 will work just fine (if I can mount the flash unit that is
).
Malcolm - Based on my manual flash settings at 68mm extension I'm pretty confident that the flash unit will be able to provide enough light up to another couple of stops (he says crossing his fingers
). I've been looking at this very versatile bracketing system developed especially for use with the Nikon R200 flash units. It looks very impressive but perhaps a bit pricey:
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What do you think?
Bruce