Back to Article List Observing Basics: Eyepieces In this episode, Senior Editor Michael E. Bakich answers one of the most common eyepiece questions he receives: What do those numbers mean? By Michael E. Bakich | Published: July 23, 2012 | Last updated on May 18, 2023 What do the numbers mean? Focal Length of your Telescope (in mm) / Focal Length of your Eyepiece (in mm) = magnification