Telescope Eyepiece & Magnification Calculator
Enter your telescope and eyepiece specs to get magnification, exit pupil, true field of view, and a suitability verdict for planets, moon, or deep sky — before spending money on glass. Nothing uploaded.
Eyepieces — click values to edit
Learn more: exit pupil, magnification, and field of view
Why exit pupil matters more than magnification
Most beginners focus on maximising magnification. But exit pupil - the beam of light exiting the eyepiece - is actually the more important parameter. An exit pupil below 1mm makes the image very dim regardless of aperture. An exit pupil above 7mm wastes light outside your dark-adapted eye. The sweet spot is 3-7mm. True field of view (how much sky you actually see) matters just as much - a 5mm eyepiece giving 200x magnification with a 0.25 degree field barely fits the full moon.
Understanding magnification and maximum useful magnification
Magnification equals telescope focal length divided by eyepiece focal length. A 1200mm focal length telescope with a 10mm eyepiece gives 120x magnification. But there is a theoretical maximum of about 50x per inch of aperture (2x per mm). Above this, the image becomes blurry regardless of eyepiece quality because you are magnifying diffraction patterns and atmospheric distortion, not real detail. The calculator flags eyepieces that exceed this limit for your telescope.
Resolving power, limiting magnitude, and true field of view
Resolving power (the smallest details you can see) depends only on aperture, not magnification - it is about 4.8 arcseconds divided by aperture in inches. Limiting magnitude (the faintest star you can theoretically see) also depends only on aperture. True field of view tells you how much sky the eyepiece actually shows - a wide-angle eyepiece (80 degree apparent field) shows more sky than a narrow eyepiece (40 degree apparent field) at the same magnification. Understanding all these parameters together helps you choose eyepieces wisely before spending money.
FAQ
What is exit pupil and why does it matter?
Exit pupil is the diameter of the beam of light leaving the eyepiece in millimetres. At the dark-adapted eye (7mm pupil) you want exit pupil of 3-7mm for the best view. Below 1mm the image is very dim. Above 7mm light spills outside your pupil. The calculator shows a red/amber/green indicator for exit pupil quality.
How do I calculate magnification for my telescope?
Magnification equals telescope focal length divided by eyepiece focal length. A 1200mm focal length telescope with a 10mm eyepiece gives 120x magnification. Enter your telescope focal length once and compare multiple eyepieces side by side.
What is the maximum useful magnification for my telescope?
The theoretical maximum is approximately 50x per inch of aperture (2x per mm). Above this the image becomes blurry regardless of eyepiece quality. The calculator flags eyepieces that exceed this limit for your telescope.