True Field of View Calculator

Calculator Inputs

Telescope Aperture (mm)

Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

Apparent Field of View (°)

Field Stop Diameter (mm)

Optical Factor

Use 1.0 for none, 2.0 for 2× Barlow, 0.63 for a reducer.

Star Declination (°)

Drift Time Across Field (seconds)

Eyepiece Series for Graph (comma separated mm)

Reset

Plotly Graph

This graph shows approximate true field of view across your eyepiece series using the apparent field and computed magnification.

Example Data Table

Scope Aperture (mm)	Scope Focal Length (mm)	Eyepiece (mm)	AFOV (°)	Magnification	Approx TFOV (°)
200	1200	25	68	48.00×	1.4167°
80	480	24	68	20.00×	3.4000°
150	750	10	82	75.00×	1.0933°
280	2800	31	82	90.32×	0.9079°

Formula Used

1) Magnification
Magnification = Effective Telescope Focal Length ÷ Eyepiece Focal Length

2) Effective Telescope Focal Length
Effective Telescope Focal Length = Telescope Focal Length × Optical Factor

3) Approximate True Field of View
True Field of View ≈ Apparent Field of View ÷ Magnification

4) Field Stop Method
True Field of View = (Field Stop ÷ Effective Telescope Focal Length) × 57.2958

5) Drift Timing Method
True Field of View = (Drift Time × cos(Declination)) ÷ 240

6) Exit Pupil
Exit Pupil = Telescope Aperture ÷ Magnification

7) Sky Area
Sky Area ≈ π × (True Field of View ÷ 2)²

How to Use This Calculator

Enter your telescope aperture and focal length.
Enter your eyepiece focal length and apparent field.
Add field stop size for a more accurate result when known.
Use optical factor for Barlows or reducers.
Enter declination and drift time if you timed a star crossing.
Submit the form to see results above the inputs.
Review comparison values, graph trends, and export the table if needed.

Frequently Asked Questions

1) What is true field of view?

True field of view is the actual sky width visible through your telescope and eyepiece combination. It is measured in degrees and helps you frame objects properly.

2) Why do approximate and field stop results differ?

The AFOV method is a practical estimate. The field stop method usually gives a better real-world answer because it uses the eyepiece’s physical light-limiting diameter.

3) When should I use drift timing?

Use drift timing when you can measure how long a star takes to cross the view. It is useful for checking actual performance without relying only on eyepiece specifications.

4) What does optical factor mean?

Optical factor adjusts the telescope focal length for accessories. A 2× Barlow doubles it, while a 0.63 reducer lowers it. Both directly change magnification and true field.

5) Why is exit pupil included?

Exit pupil helps you judge brightness and comfort. Larger values create brighter wide-field views, while very small values can dim the image and emphasize seeing conditions.

6) Does larger apparent field always mean larger true field?

Not always. Apparent field helps, but true field also depends on magnification and telescope focal length. A wide eyepiece in a long focal length telescope can still show a modest sky area.

7) Which result should I trust most?

If you know the field stop, trust that method first. If you timed a star drift carefully, compare it too. Use the AFOV method as a fast planning estimate.

8) Can this calculator help with object framing?

Yes. The calculated field shows whether large objects, star fields, or lunar regions will fit. The graph also helps compare eyepieces before choosing one outdoors.