Telescope Magnification
Calculator
Results
- Magnification
- 120
- Exit pupil (mm)
- 1.666666
- Focal ratio (f/number)
- 6
Astronomy results
| Magnification | 120 |
| Exit pupil (mm) | 1.666666 |
| Focal ratio (f/number) | 6 |
formula-map diagram
- Magnification
- 120
- Exit pupil (mm)
- 1.666666
- Focal ratio (f/number)
- 6
Astronomical relationship
Formula
M = f_telescope / f_eyepiece= 120
Note
This result is a simplified model: it applies the displayed textbook formula to the values you entered, assuming ideal spherical bodies, circular orbits, blackbody radiation and perfect optics, and ignoring atmospheric seeing, relativistic corrections beyond those stated, cosmological models and measurement uncertainty. Use published ephemerides and catalogue data for real observations.
More in Astronomy and space
See all →Frequently asked questions
How is a telescope's magnification calculated?+
Magnification equals the telescope's focal length divided by the eyepiece's focal length. A telescope with an 1,000 mm focal length paired with a 10 mm eyepiece gives 100x magnification.
Why does using a shorter focal length eyepiece increase magnification?+
Because magnification is an inverse relationship with eyepiece focal length, a shorter eyepiece focal length produces a larger result for the same telescope. This is why observers swap in short eyepieces, like 6mm, for high-power views of the Moon or planets.
Is more magnification always better for viewing?+
No, this is a common misconception — every telescope has a practical maximum useful magnification, roughly 50x its aperture in inches (or about 2x its aperture in millimeters). Beyond that limit, the image just gets larger, dimmer, and blurrier rather than showing more detail.
Why does high magnification make images dimmer and shakier?+
Magnification spreads the same amount of collected light over a larger apparent image, reducing brightness, and it also magnifies atmospheric turbulence and any mount vibration. This is why deep-sky objects like galaxies are often better viewed at lower magnification, where the image stays bright and steady.
Does the telescope's aperture affect the magnification formula?+
No, aperture (the diameter of the main lens or mirror) doesn't appear in the magnification formula at all — magnification depends only on focal lengths. Aperture instead determines light-gathering power and resolving power, which is why a wide but short-focal-length telescope isn't automatically capable of high, sharp magnification.