mpenza said:"X" is not that useful or descriptive for SLR lenses. I was using a prime lens, so technically it's 1x. The "35mm equivalent" focal length is 380mm.
Ah, I was actually asking you what's the resulting magnification factor you got and not the focal length, as you didn't mention what's the teleconvertor you used and I have no idea how many times magnification the B300 does. Sorry if the question is confusing...
Anyway, I fully agree with you that X is not a useful descriptive term for lenses at all (SLR or not). How the heck can anybody even know what a 10X lens does? A 2-20mm lens is 10X, so is a 50-500mm lens. I swear, the X branding is only used by manufacturers because X sounds cool, and also because people think the more X the better, so 20X confirm must be better than 2.85X of a 70-200mm f2.8...