Macro extension tubes?

-depends on the lens. e.g. symmetrical lens designs may work better than others. Zooms and wide-angles will crap out eventually, quality and focal plane wise. Try a standard prime or a dedicated macro lens; if you do find degradation, either buy a cheap reversing ring which lets you put the lens on backwards (but you won't be able to focus anymore) or try a lens from an enlarger. The other problem is of course the your plane of focus gets too close, so then a longer lens is needed. I'm sure there are webpages dealing with this; I have books on the stuff somewhere. All the best!