Telescope Magnification Calculator
Enter your telescope and eyepiece focal lengths to find the magnification. Add the aperture for the maximum useful power, exit pupil and focal ratio too.
How to use the telescope magnification calculator
- Enter the telescope focal length in millimetres.
- Enter the eyepiece focal length in millimetres.
- Optionally add the aperture for exit pupil and focal ratio.
Examples
1000 mm scope with a 10 mm eyepiece
scope = 1000 mm, eyepiece = 10 mm
100x magnification
Adding a 100 mm aperture
scope = 1000 mm, eyepiece = 10 mm, aperture = 100 mm
max useful 200x, exit pupil 1 mm, f/10
Frequently asked questions
How do you calculate telescope magnification?
Divide the telescope's focal length by the eyepiece focal length. A 1000 mm scope with a 10 mm eyepiece gives 1000 / 10 = 100x.
What is the maximum useful magnification?
A common rule of thumb is about 2x the aperture in millimetres. A 100 mm aperture tops out near 200x before the image gets dim and fuzzy.
What is the exit pupil?
The exit pupil is the width of the light beam leaving the eyepiece, equal to aperture divided by magnification. Lower power gives a wider, brighter beam.
What is the focal ratio (f-number)?
It is the telescope focal length divided by its aperture. A 1000 mm scope with a 100 mm aperture is f/10. Lower f-numbers give brighter, wider fields.
Why does more magnification not always help?
Beyond the maximum useful power the image grows but gathers no extra light or detail, so it appears dim and blurry. Aperture sets the real limit.
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