Telescope Magnification Calculator
Calculate telescope magnification, field of view, and exit pupil from focal lengths.
Find the best eyepiece for your telescope.
Telescope Optics
Telescope magnification is determined by the focal lengths of the telescope and eyepiece.
Key formulas:
Magnification = Telescope Focal Length / Eyepiece Focal Length
Exit Pupil (mm) = Telescope Aperture / Magnification
Maximum Useful Magnification ≈ 2 × Aperture (mm)
Minimum Useful Magnification ≈ Aperture (mm) / 7
Field of view:
True FOV = Apparent FOV of Eyepiece / Magnification
Understanding exit pupil:
- 7 mm = maximum human pupil size (dark-adapted)
- 5-7 mm = best for deep-sky objects (galaxies, nebulae)
- 2-4 mm = good for general viewing
- 1-2 mm = best for planets and the Moon
- Below 1 mm = usually too dim
Eyepiece apparent fields of view:
- Plossl: 50-52°
- Wide-angle: 60-68°
- Ultra-wide: 82-100°