THE OBSERVING TOOLKIT
Eyepiece calculator
Calculate telescope magnification, exit pupil, and approximate true field with formulas and a worked example.
This is a worked example. Enable JavaScript to change the inputs and calculate your own setup; the formulas below work on paper too.
Your setup
Use millimeters for the first three measurements. No Barlow lens or focal reducer is assumed.
YOUR VIEW, IN NUMBERS
- Magnification
- 40.0×
- Exit pupil
- 2.50 mm
- Approximate true field
- 1.25°
Calculated geometry, not a prediction of image quality. Seeing, alignment, focus, and eyepiece design affect the actual view.
What the numbers mean
Magnification = telescope focal length ÷ eyepiece focal length. Exit pupil = aperture ÷ magnification. Approximate true field = apparent field ÷ magnification. Keep length measurements in the same unit.
For the example above, 800 ÷ 20 = 40×. The exit pupil is 100 ÷ 40 = 2.5 mm, and the estimated true field is 50 ÷ 40 = 1.25°. Change the eyepiece to 10 mm to compare the result: 80×, 1.25 mm, and 0.625°.
Put the comparison to work
Record the results for two eyepieces you own, then compare them on the same object. Note which view is easier to center and which reveals a detail consistently. A larger magnification number alone does not establish a better view.
The field estimate is approximate; a field-stop calculation is more precise when that specification is available. These results are not an equipment compatibility check or a maximum useful magnification recommendation.
Read the eyepiece field guide →Formula reference: Celestron’s eyepiece guide.