Originally posted by normhead Or you can just learn an FF is one stop shallower DoF than the same lens on APS-c. (And not learn equivalence.)

No one is saying equivalence is worthless. But, it's possible to learn everything you need to know, other ways. Equivalence is overshadowed by more useful concepts.

As I said, IMHO, equivalence is make a work project.

No matter what method you use, if you get the relations correct, you have learnt the equivalency between the formats.

The mathematical method is usually the quickest an most simple way to learn about equivalency, because the calculations are so simple.

If you understand that APS-C give you one extra stop DOF at the same FOV as FF, and for the same FOV you need to divide focal length by 1.5 for APS-C.

If you understand how exposure work, then it is basically everything you need to know about equivalency between APS-C and FF.

It is so easy that photographers that use multiple formats do not need to think about it after a few days.