Telescope Magnification Calculator
Work out a telescope's magnification, max useful power, and exit pupil from its focal lengths and aperture.
Magnification is just the scope's focal length divided by the eyepiece's. Swap in a shorter eyepiece for more power — but only up to a point: past about twice the aperture in millimeters, the image turns dim and mushy rather than more detailed.
How it works
A telescope's magnification is the telescope's focal length divided by the eyepiece's focal length. A 1200 mm scope with a 10 mm eyepiece gives 120×. Swap to a 6 mm eyepiece and you jump to 200×.
More power isn't always better. The tool also shows the max useful magnification — about twice your aperture in millimeters — beyond which the image just gets dim and fuzzy instead of showing more detail.
It reports the exit pupil too: the width of the beam of light leaving the eyepiece, found by dividing aperture by magnification. Around 2 to 3 mm is a comfortable, high-contrast range for most targets.
Frequently asked questions
How do I calculate telescope magnification?
Divide the telescope's focal length by the eyepiece's focal length, both in millimeters. A 900 mm telescope with a 25 mm eyepiece gives 900 ÷ 25 = 36×.
What is the maximum useful magnification?
A common rule of thumb is about 2× the aperture in millimeters. A 150 mm scope tops out near 300×; push past that and the view turns faint and blurry rather than sharper.
What is exit pupil and why does it matter?
It's the diameter of the light beam leaving the eyepiece, equal to aperture divided by magnification. A large exit pupil gives a bright, low-power view; a small one gives a dim, high-power view. Matching it to your eye keeps the image crisp.