File:Orthographic camera distance focal length.gif - Wikimedia Commons
light - the eye -how lenses work
Lens Calculator - LUCID Vision Labs
Proud Business Productions - How To Make Your Phone Footage Stand Out
This gif explains how changing focal length impacts a portrait
Is camera lens value basically the same as FOV? - User Feedback - Developer Forum
Best lens for headshots - why lens choice is critical for good portraits
Safwan AhmedMia on Twitter: "For those who don't understand focal length and keep saying "You don't need to use the Telephoto Lens for Portraits, just move closer!" 🤦🏽♂️ This GIF from @parkerwalbeck