There seems to be some confusion here on what a zone is. In Ansel Adam's Zone System, the term "zone" is used only when referring to film exposure. When referring to negative densities or print densities, he uses the term "values", not zones. Adams says that zones are one stop (an exact doubling or halving of exposure, or change of .3 subject reflection density units) apart. His system is based around this central idea. "Values" can be any distance apart based on many factors.
In sensitometry, a change of .3 density units on a negative is a doubling or halving of density, but I don't believe I have ever heard this use correspond to the term "stop". I believe a stop refers only to a doubling or halving of exposure, not a doubling or halving of film density.
Originally Posted by Stephen Benskin