Bummer! Not sure why I was thinking the effective aperture was a stop and a bit down with the tubes on regardless of lens used...does make more sense the way you put it

Just aswell for AE!

However, doubling focal length with tubes should still give better quality than doubling with a TC....tubes just magnify "flaws" (abberations?) already in the lens compared to a TC which magnifies the same flaws and also adds some new ones. Hope I at least got that right