Free interactive tool
Telescope Magnification Calculator
Scope focal length ÷ eyepiece = power — and the aperture sets the honest limit.
Build a magnification from two focal lengths
Divide the telescope's focal length by the eyepiece's. Then check it against what the aperture can honestly resolve.
Magnification is free to compute and expensive to support: past roughly twice the aperture in millimeters, extra power magnifies blur instead of detail.
- Hit exactly 100× with two different eyepiece choices.
- Push past the useful limit and read what the model warns.
Magnification is the telescope's focal length divided by the eyepiece's — a 900 mm scope with a 9 mm eyepiece gives 100×. But power past roughly twice the aperture in millimeters just magnifies blur.
Try eyepiece combinations before buying them, and see when a shorter eyepiece stops adding detail.
What magnification to see Saturn's rings?
They appear at ~50× and look pleasing at 100–150× in steady air.
Why does my image go dim at high power?
The same collected light is spread over more apparent area — aperture, not eyepiece, buys brightness.