Telescope Magnification Calculator Formula
Understand the math behind the telescope magnification calculator. Each variable explained with a worked example.
Formulas Used
Magnification
magnification = focal_obj / focal_eyeVariables
| Variable | Description | Default |
|---|---|---|
focal_obj | Objective Focal Length(mm) | 1200 |
focal_eye | Eyepiece Focal Length(mm) | 25 |
How It Works
Telescope Magnification
M = f_objective / f_eyepiece
Swapping eyepieces changes magnification. The maximum useful magnification is roughly 2× per mm of aperture (e.g., 300× for a 150 mm telescope).
Worked Example
1200 mm scope with a 25 mm eyepiece.
- 01M = 1200 / 25 = 48×
- 02Objects appear 48 times larger (linearly).
Frequently Asked Questions
Is higher magnification always better?
No. Beyond the maximum useful magnification (about 2× per mm of aperture), the image becomes dim and blurry. Atmospheric turbulence also limits useful magnification.
What magnification do I need for planets?
Planets benefit from 100-250×. Jupiter's cloud bands and Saturn's rings are visible at 100×.
How does a Barlow lens work?
A Barlow is a diverging lens placed before the eyepiece that multiplies the effective focal length, typically by 2× or 3×, doubling or tripling magnification.
Ready to run the numbers?
Open Telescope Magnification Calculator