That's a good article. The article I posted previously gives a more technical explanation, that is also interesting, but the DOFMaster is an interesting and more intuitive approach. I had not thought about there being two components to the difference: crop factor and subject distance.
Since an FX camera isn't in my near future, this isn't immediately applicable to me. However, I did learn something that had never occurred to me before that will be: adding a TC decreases depth of field. I had always assumed that the increase in f-stop offset the additional focal length, but that isn't true. I still have about one stop less DOF when adding a TC.
"Something really clever or profound should go here...I have nothing "