Telescope Magnification by Eyepiece & Focal Length
Calculate telescope magnification from focal length and eyepiece focal length, plus the maximum useful magnification for your aperture.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Magnification = Telescope Focal Length ÷ Eyepiece Focal Length
Divide the telescope's focal length (mm) by the eyepiece focal length (mm) to get magnification. Exit pupil = aperture ÷ magnification. Maximum useful magnification ≈ 2× aperture (mm). Limiting magnitude ≈ 2.7 + 5 × log₁₀(aperture mm).
Worked Examples
Example 1: 8-inch Dobsonian with 25mm Eyepiece
Problem:Calculate the magnification and exit pupil for a 200mm f/6 Dobsonian (1200mm FL) with a 25mm Plössl eyepiece.
Solution:Magnification = 1200 / 25 = 48×\nExit pupil = 200 / 48 = 4.17mm\nTrue FOV ≈ 50° / 48 = 1.04°\nMax useful mag = 2 × 200 = 400×
Result:48× magnification, 4.17mm exit pupil — excellent for deep-sky observing
Example 2: Planetary Viewing at High Power
Problem:Same telescope with a 5mm eyepiece for planetary observation.
Solution:Magnification = 1200 / 5 = 240×\nExit pupil = 200 / 240 = 0.83mm\nTrue FOV ≈ 50° / 240 = 0.21°\nWithin max useful of 400×
Result:240× magnification, 0.83mm exit pupil — good for planets and Moon
Frequently Asked Questions
What is telescope magnification and how is it calculated?
Magnification (power) is the number of times larger an object appears through the telescope compared to the naked eye. It is calculated by dividing the telescope's focal length by the eyepiece's focal length: Magnification = Telescope FL ÷ Eyepiece FL. For example, a 1200mm telescope with a 25mm eyepiece gives 48× magnification. You can change magnification by swapping eyepieces.
What is the maximum useful magnification?
The maximum useful magnification is approximately 2× the aperture in millimeters (50× per inch). Beyond this, the image becomes dim and blurry due to diffraction limits. For a 200mm (8-inch) telescope, the max useful magnification is about 400×. Atmospheric seeing conditions often limit practical magnification to 200-300× regardless of telescope size.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy