Photography focusing problem

Don't talk nonsense if you don't understand the top three. It's embarrassing.

There is a saying. Many books used to say that. It's ok to use, there is no deviation, at least I can't see it.

This is mainly used for the early manual focusing lens, which can see the scene more clearly and focus more easily at the telephoto end. Now it's all autofocus, which can focus accurately in all focal lengths, so it's not mentioned in the book now, and some novices don't know it. Today's autofocus cameras don't need to focus at the telephoto end.

As for the optical principle, I don't know.