I got a 12mm extension tube (from Canon), and just got my good lens back (fixed under warranty).
So I went out to try some flowers with off-camera flash at dusk, see what I could get.
I was shocked to see some really nasty CA at one corner. I don't normally have any problems with this lens (EF-S 17-55). I'm guessing that putting it out on an extension tube means that all the high-tech computer calculations they do for the lens get screwed up?
Here's the full frame, no PP except capture sharpening:
and a crop of the upper-right:
You can really see it on the edge between the last white petal and the dark background, also some between the nearer and farther petals.
So, do I just try to center and crop with the extension tube? PP it out? Or do I have to spring for a macro lens?