[Brightcolours]

For macro use, I never understood the often repeated claims that extension tubes don't degrade image quality like a front mounted "close up filter" does, since you are pushing the lens outside its design intent. Anyway, when using the 50mm f/1.8 II I had noticed you get massive field curvature when you use extension tubes. I didn't pay attention to what other nasties it may or may not have added too, but wouldn't be surprised if there's more.

Extension tubes do not degrade "image quality" with well designed lenses. That a 50mm f1.8 is not optimal for certain uses is not the fault of extension tubes... Just the extension tubes show the shortcoming of that particular lens. There are lenses with which you can use extension tubes, with perform just fine.