Optics & gearDefinedTerm
Focal length
The distance from a telescope's objective to the focused image.
Sets the image scale together with the eyepiece focal length.
Used in these guides
- Refractor vs. Reflector vs. Catadioptric: Optics, Honestly ComparedOptics & gear
- Your First Telescope: Why Aperture Beats MagnificationOptics & gear
- Eyepieces and Focal Ratio: The Math That Sets Your ViewOptics & gear
- Astrophotography with What You Have: Phone and TripodImaging the sky
- Galileo's Telescope: Four Moons That Changed EverythingHow we learned where we are