What is the Telescope Magnification Calculator?
Calculate telescope magnification from focal lengths with this free telescope magnification calculator. Get exit pupil, true field of view, resolving power, and light grasp. Find the right eyepiece or telescope focal length for your target magnification. Runs entirely in your browser with no signup.
How to use the Telescope Magnification Calculator
- Choose Calculate Magnification, Find Eyepiece FL, or Find Telescope FL mode.
- Enter telescope and eyepiece focal lengths in millimeters.
- Optionally add aperture and eyepiece apparent field of view.
- Click Calculate to get magnification and optical metrics.
- Copy results and step-by-step formulas.
Common use cases
- Finding magnification for a 1200 mm refractor with a 25 mm eyepiece
- Choosing a 5 mm eyepiece for 200× on a 1000 mm scope
- Checking exit pupil and light grasp for an 8-inch SCT
Frequently asked questions
- How do you calculate telescope magnification?
- Magnification M = telescope focal length ÷ eyepiece focal length. A 1200 mm scope with a 25 mm eyepiece gives 48× magnification.
- What is exit pupil?
- Exit pupil = aperture diameter ÷ magnification. It is the width of the light beam leaving the eyepiece. Values above ~7 mm waste light because the human eye cannot open wider in the dark.
- How is true field of view calculated?
- True FOV ≈ eyepiece apparent field of view ÷ magnification. A 50° eyepiece at 100× gives about 0.5° of sky.
- What is the Dawes resolving power?
- Theoretical resolving power ≈ 116 / D(mm) arcseconds. An 8-inch (203 mm) telescope can resolve about 0.57 arcseconds under ideal conditions.
