Field of View Calculator Formula
Understand the math behind the field of view calculator. Each variable explained with a worked example.
Formulas Used
Magnification
magnification = focal_obj / focal_eyeTrue Field of View
tfov = afov / (focal_obj / focal_eye)True FOV (arcmin)
tfov_arcmin = afov / (focal_obj / focal_eye) * 60Variables
| Variable | Description | Default |
|---|---|---|
afov | Apparent FOV of Eyepiece(°) | 52 |
focal_obj | Objective Focal Length(mm) | 1200 |
focal_eye | Eyepiece Focal Length(mm) | 25 |
How It Works
True Field of View
TFOV = AFOV / Magnification
The apparent field of view (AFOV) is a property of the eyepiece design (e.g., 52° for Plossl, 68° for wide-angle, 82° for ultra-wide). Dividing by magnification gives the actual sky patch visible.
The full Moon is about 0.5° across, useful as a reference.
Worked Example
52° AFOV eyepiece at 48×.
afov = 52focal_obj = 1200focal_eye = 25
- 01Magnification = 1200 / 25 = 48×
- 02TFOV = 52 / 48 = 1.083° = 65.0 arcmin
- 03This fits about two full Moons side by side.
Frequently Asked Questions
What AFOV do common eyepiece types have?
Plossl: 50-52°, Erfle/wide-angle: 65-68°, Nagler/ultra-wide: 82°, Ethos: 100°.
How do I find objects in a narrow FOV?
Use a lower magnification eyepiece (wider TFOV) to locate objects, then switch to higher magnification for detail.
Does the true FOV change with a Barlow?
Yes. A 2× Barlow doubles the magnification, halving the true field of view.
Ready to run the numbers?
Open Field of View Calculator