True Field of View Calculator

• Magnification: M = F_eff / f_ep
• Effective focal length: F_eff = F_scope × factor
• AFOV estimate: TFOV ≈ AFOV / M
• Field stop method: TFOV ≈ (D_fs / F_eff) × (180/π)
• Sensor geometry: FOV = 2×atan(d / (2F_eff)) × (180/π)

Notes: The AFOV estimate is convenient but depends on how accurately AFOV is stated and how the eyepiece behaves. Field stop values, when provided by the maker, often produce closer real-world coverage.

1. Select a calculation method that matches your equipment.
2. Enter your focal length values and choose the proper units.
3. Set the optical factor for Barlow lenses or reducers.
4. For eyepieces, provide AFOV and eyepiece focal length.
5. If available, add field stop diameter for a refined result.
6. Click Calculate to see degrees, arcminutes, and more.
7. Use the export buttons to save your results as files.

1) What true field of view actually means

True field of view (TFOV) is the real slice of sky you can see at once. It is usually expressed in degrees, arcminutes, or arcseconds. For perspective, the full Moon spans about 0.5° (30 arcmin), while the Pleiades cover roughly 2°. Knowing TFOV helps you decide if an object will fit, or if you need a wider setup.

2) Eyepiece estimate and why it is popular

The fastest TFOV estimate uses the eyepiece’s apparent field of view (AFOV) and your magnification. Magnification is telescope effective focal length divided by eyepiece focal length. Many common eyepieces have AFOV values around 40°, 50–52°, 60–70°, or 82–100°. This estimate is excellent for quick planning and comparing eyepieces in the field.

3) Field stop method for tighter accuracy

If you know the eyepiece field stop diameter, you can compute TFOV more directly. Typical maximum field stops are about 27 mm for 1.25-inch formats and about 46 mm for 2-inch formats. With a 1200 mm telescope and a 27 mm field stop, TFOV is near 1.29°. That can differ from the AFOV estimate, especially with complex designs.

4) Sensor geometry for camera framing

Camera framing uses sensor width, height, and the effective focal length. The calculator outputs horizontal, vertical, and diagonal fields using an arctangent formula. For example, a 22.3×14.9 mm sensor at 400 mm focal length yields roughly 3.19° by 2.13°. This is ideal for mosaics, composition, and matching targets to your camera.

5) Optical factor: Barlow and reducer effects

Optical factor scales focal length before any TFOV computation. A 2× Barlow doubles effective focal length, doubles magnification, and roughly halves TFOV. A 0.8× reducer shortens focal length and increases TFOV by about 25%. Entering the factor makes the output reflect your real optical train.

6) Degrees, arcminutes, and arcseconds

One degree equals 60 arcminutes, and one arcminute equals 60 arcseconds. Wide-field views for star clusters may be 1–4°. Planetary and double-star work often uses much smaller effective fields, where arcminutes matter. The calculator converts all outputs so you can communicate framing precisely.

7) Using TFOV to choose targets

Compare TFOV to the object’s angular size to predict fit. If your TFOV is 1°, a 3° object will require a mosaic or a shorter focal length. If your TFOV is 2°, many large clusters and nebulae become easier to frame. This planning step saves time during observing sessions.

8) Practical tips for better inputs

Use manufacturer field stop values when possible, and keep units consistent. AFOV values are sometimes optimistic, so treat the AFOV method as an estimate. For cameras, confirm sensor dimensions from the datasheet. When in doubt, test on a star field and refine your inputs for repeatable results.

1) Which TFOV method should I trust most?

Field stop and sensor geometry are usually most reliable. The AFOV estimate is great for quick comparisons, but accuracy depends on how true the stated AFOV is for your eyepiece.

2) Why does my AFOV-based TFOV differ from the field stop TFOV?

AFOV is often rounded, marketing-based, or varies with eyepiece design. Field stop calculations use a physical aperture dimension, so they can better reflect the true limiting field.

3) What optical factor should I enter for a reducer or Barlow?

Enter the multiplier of your effective focal length. Use 2 for a 2× Barlow, 0.8 for a 0.8× reducer, or 1 if you are using no additional optics.

4) Does TFOV change when I change aperture?

Not directly. TFOV is mainly set by effective focal length and the field limiter (field stop or sensor size). Aperture affects brightness and exit pupil, not the geometric field.

5) What is a good TFOV for deep-sky observing?

Many observers enjoy 1–2° for general deep-sky work, while 3–5° is excellent for very large targets and star fields. Your preferred TFOV depends on your targets and sky conditions.

6) How do I measure field stop if it is not listed?

Some users estimate it from eyepiece internals, but measurements can be inaccurate. A safer approach is drift timing on a star near the celestial equator to infer the true field.

7) Can I use this for binoculars?

Yes, if you know the binocular’s true field directly, you can interpret it in degrees and arcminutes. If you know AFOV and magnification, the AFOV method can also give a quick estimate.