The real meaning of back focus. I first time I heard the term back focus was in the 1970s when I worked for a company that had a lens repair dept.
The first thing you need to know is depth of focus. "Depth of focus ... is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus. It can be viewed as the flip side of depth of field, occurring on the opposite side of the lens." http://en.wikipedia.org/wiki/Depth_of_focus
Now that you know about depth of focus you can start to understand why back focus is important. If the back focus between the lens and the image sensor/film is not is not correct you will have a focus problem. So if you have a back focus problem on a still camera it needs to go in for repair, or if using pro video cameras you can adjust back focus in the field. Here's an article on back focus as it relates to video http://www.bandprodigital.com/cgibin/eDatCat/BPDstore.cgi?user_action=detail&catalogno=TEK-BF
So if you tell a lens tech that you have a back focus problem he will think that there is a flange focal distance problem. Which is not what you mean at all.
A quote fro "Alice in Wonderland": 'When I use a word,' Humpty Dumpty said, in a rather scornful tone,' it means just what I choose it to mean, neither more nor less.'
'The question is,' said Alice, 'whether you can make words mean so many different things.'
'The question is,' said Humpty Dumpty, 'which is to be master - that's all.'
Humpty Dumpty would be right at home in a 'net forum.
Lots of stuff in life like that. Remember CNN's use of the word "factoid"? They used it to mean an interesting but true fact. However it actually means something appearing to be a fact but is actually false, e.g., inaccurate information that is presented in the press as factual and is then accepted as true because of repetition. Leo Fender labeled the tremolo switch on his amps as vibrato and to this day many Americans mix the two up. This ain't evolving language. It's more akin to confusion.
But, yeah, the phrase "back focus" is both defined and used in early Canon Lens Work books in the true optical definition, not in the recent internet usage.
