Telescope Magnification Calculator
Magnification from an eyepiece, with the exit pupil, true field of view, useful maximum and limiting magnitude.
Results
What this tool does
The exit pupil is the width of the light beam leaving the eyepiece: above about 7 mm some of it misses your pupil and is wasted, and below 0.5 mm floaters in your own eye start to show. Somewhere between 2 and 3 mm is where most deep-sky observing is comfortable.
Formula
magnification = distance pipe focal ÷ distance eyepiece focal
Variables
| Symbol | Meaning | Unit |
|---|---|---|
fo | Telescope focal length | mm |
fe | Eyepiece focal length | mm |
ap | Aperture | mm |
af | Eyepiece apparent field | ° |
M | Magnification | × |
EP | Exit pupil | mm |
FV | True field of view | ° |
MM | Magnification | × |
LM | Limiting magnitude | — |
FR | Focal ratio | — |
Worked example
- Telescope focal length1200 mm
- Eyepiece focal length25 mm
- Aperture150 mm
- Eyepiece apparent field52 °
- Magnification48.00 ×
- Exit pupil3.125 mm
- True field of view1.0833 °
- Magnification300 ×
- Limiting magnitude12.880
- Focal ratio8.000
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.
Frequently asked questions
Why is more magnification not better?
Because magnification spreads the same collected light over a bigger image, so the view gets dimmer as it gets larger, and the wobble of the atmosphere gets magnified too. The practical ceiling is about two times the aperture in millimetres, shown as the fourth result, and on most nights the air itself limits you well below that. Aperture, not magnification, is what shows fainter objects.