This question featured a quote from @Itai's Neocamera site, and the original text there had a math error. That is now corrected.
It's a really good basic terminology/lens question, though, and at this point the error-correction parts of the answers are just noise — not useful at all to future readers who might discover or be pointed to the answers.
I'm inclined to edit the question and remove the error, and do the same thing for the answers. Opinions on that, anyone?