Telescope Magnification Calculator

Telescope magnification determines how large a celestial object appears through the eyepiece compared to the naked eye.
Magnification is only half the story, though. Exit pupil and true field of view decide whether the image is bright enough and wide enough to be worth looking at.

Formulas: Magnification = Telescope Focal Length ÷ Eyepiece Focal Length Exit Pupil = Telescope Aperture ÷ Magnification True Field of View = Apparent Field of View (AFOV) ÷ Magnification Maximum Useful Magnification ≈ Aperture (mm) × 2 Minimum Useful Magnification ≈ Aperture (mm) ÷ 7

What each variable means:

• Telescope Focal Length: stamped on the tube, typically 400–2000mm for consumer scopes.
• Eyepiece Focal Length: marked on the eyepiece barrel; smaller number = higher magnification.
• Aperture: the diameter of the objective lens or mirror; the most important spec for light gathering.
• Exit Pupil: the diameter of the light beam exiting the eyepiece into your eye. Ideal range: 3–7mm for nighttime viewing. Below 1mm = dim, hard-to-use image.

Worked example: Telescope: 8-inch (203mm) Dobsonian with 1200mm focal length. Eyepiece: 25mm Plössl.

Magnification = 1200 ÷ 25 = 48× Exit Pupil = 203 ÷ 48 = 4.2mm ✓ (bright, comfortable) Max useful magnification = 203 × 2 = 406× (requires excellent seeing conditions) Practical limit on most nights: 150–200×

Switching to a 6mm eyepiece: Magnification = 1200 ÷ 6 = 200× Exit Pupil = 203 ÷ 200 = 1.0mm (dim, so use it only on bright targets like the Moon or the planets)

Best magnification by target:

• Wide nebulae and star clusters: 20–50× (large exit pupil, wide field)
• Globular clusters: 100–200×
• Moon and planets: 150–300× (limited by atmospheric seeing)
• Double stars: 200–400×

Atmospheric seeing, the turbulence in the air above your head, is the real limit on magnification.
On an average night anything beyond 150 to 200× gives you a blurry, shimmering image no matter how good the telescope is. The 406× this page calculates for an 8-inch scope is a ceiling the sky grants you a few nights a year, not a target.