WHY does aperture change depth of focus???

After getting some old cameras I was reintroduced to the idea of depth of field/focus. As a physics teacher my mind immediately started wondering WHY this worked. I found plenty of videos describing HOW adjusting f-stop affected depth of focus... but no real explanation as to why this works the way it works. So, I made a video. It's sort of a boring video (I feel for my students now). But I think it does a decent job of describing the optics of depth of focus. Optics App: https://phydemo.app/ray-optics/ Other good videos on the HOW:    • A Simple Guide to Depth of Field      • Understanding Depth of Field: How Aperture...   Chapters 0:00 Intro 00:51 Disclaimer 01:08 The HOW 01:50 The WHY 02:30 Lens Simulation 05:30 Two Sources 07:46 Recap