A worked example
At 1,000 mm focal length, a 25 mm / 60° eyepiece gives approximately 1.5°, while a 10 mm / 60° eyepiece gives 0.6°.
Eyepiece fields
Compare two eyepieces on the same telescope with an approximate true-field diagram.
YOUR LITTLE WORKBENCH
Eyepiece fields
Start with an example and change it to your setup. Results update as you type.
Calculation method+
Approximate true field = apparent field ÷ magnification. The field-stop method can differ.
YOUR LIVE RESULT ✦
Approximate true field · A
1.5 °- Approximate true field · B
- 0.6 °
- Magnification · A
- 40 ×
- Magnification · B
- 100 ×
- Field diameter ratio A / B
- 2.5
The star pattern is illustrative, not a sky atlas. Apparent-field division is an approximation, especially for wide-angle eyepieces.
THE METHOD / 01
Know what goesinto your result.
Approximate true field = apparent field ÷ magnification. The field-stop method can differ.
Keep the assumptions clear
The star pattern is illustrative, not a sky atlas. Apparent-field division is an approximation, especially for wide-angle eyepieces.
A FEW USEFUL ANSWERS
Good to know.
Are my inputs uploaded?
No. Calculations run locally in your browser. The host still receives normal requests for website pages and assets.
Can I keep my result?
Use the download controls for the current tool or print from your browser. Typing scores and camping plans can also be saved locally; see the privacy page for details.
Do I need an account?
No sign-up, paid API or subscription is required. Both language versions use the same calculation logic.
SKYKIND / TOOLKIT
A little help for every step.
01
Magnification
Calculate magnification, exit pupil and effective focal ratio from your optical setup.
Open tool03
Camera field
Calculate rectangular camera field of view and compare it with an entered angular target size.
Open tool04
Pixel scale
Convert pixel size and focal length to arcseconds per pixel, with binning and a user-entered seeing value.
Open tool