What is TFOV and why does it matter?

TFOV (True Field of View) is the actual patch of sky visible through an eyepiece or camera sensor. A wider TFOV lets you frame large objects like the Andromeda Galaxy (3.2°) or the Pleiades (2°) in a single view without mosaicking.

How do I calculate magnification?

Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. A 1200 mm telescope with a 25 mm eyepiece gives 48× magnification. Higher FL eyepieces give lower magnification and wider TFOV.

What is a good exit pupil for visual observing?

For dark-sky deep-sky viewing, an exit pupil of 5–7 mm is ideal (matches the fully dark-adapted eye). For planetary detail, use 1–3 mm for high contrast. Exit Pupil = Eyepiece Focal Length ÷ Telescope f/ratio.

Can I use this for astrophotography planning?

Yes. Switch to Camera mode, enter your sensor dimensions (from the camera data sheet), and verify if your target object fits within the computed TFOV at your telescope's focal length. Combine with a star chart for final framing.

What will I see through my telescope?

Enter your telescope aperture, focal length, and eyepiece or camera details above. The Telescope FOV Calculator on fastool.io instantly shows your True Field of View in degrees, magnification, and exit pupil. For example, a 200mm f/5 telescope with a 25mm Plössl eyepiece gives 40× magnification and roughly 1.3° TFOV — enough to frame the full Moon. Switch to Camera mode for astrophotography FOV planning.

Telescope FOV Calculator

Plan your eyepiece & camera combinations — compute true FOV, magnification, and exit pupil instantly in your browser

Zero Upload — all calculations run in your browser

Showing example: 10″ Dobsonian (254 mm / f4.7) + Plössl 25 mm. Enter your own values to get your result.

Telescope

Aperture (mm)

Focal Length (mm)

Eyepiece

Preset Eyepiece

Eyepiece Focal Length (mm)

AFOV — Apparent Field of View (°)

Results

Magnification: 48.0×
True FOV (TFOV): 1.42°
Exit Pupil: 5.3 mm
f/Ratio: f/4.7

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How Telescope FOV Works

True Field of View (TFOV)

TFOV = AFOV ÷ Magnification. It is the actual swath of sky visible through your eyepiece. Lower magnification means a wider TFOV — ideal for open clusters and large nebulae like M45 or M42.

Sky & Telescope: TFOV Guide

Exit Pupil & Eye Relief

Exit Pupil (mm) = Eyepiece FL ÷ f/ratio. A 7 mm exit pupil matches the dark-adapted eye's maximum dilation. Too small (<1 mm) makes the image dim; too large wastes aperture.

Cloudy Nights: Exit Pupil

Useful Magnification Range

Minimum useful magnification ≈ Aperture(mm) ÷ 7; maximum ≈ Aperture(mm) × 2. Beyond maximum, the image appears 'empty' — atmospheric seeing limits further detail before optics do.

Cloudy Nights: Magnification

Astrophotography FOV

TFOV(°) = 2 × arctan(sensor size ÷ (2 × focal length)). Match your TFOV to your target: M31 ≈ 3.2°×1°, Orion Nebula ≈ 1°, Moon ≈ 0.5°. Use Camera mode to plan imaging setups.

AstroBin Imaging Reference

Frequently Asked Questions

What is TFOV and why does it matter?: TFOV (True Field of View) is the actual patch of sky visible through an eyepiece or camera sensor. A wider TFOV lets you frame large objects like the Andromeda Galaxy (3.2°) or the Pleiades (2°) in a single view without mosaicking.
How do I calculate magnification?: Magnification = Telescope Focal Length ÷ Eyepiece Focal Length. A 1200 mm telescope with a 25 mm eyepiece gives 48× magnification. Higher FL eyepieces give lower magnification and wider TFOV.
What is a good exit pupil for visual observing?: For dark-sky deep-sky viewing, an exit pupil of 5–7 mm is ideal (matches the fully dark-adapted eye). For planetary detail, use 1–3 mm for high contrast. Exit Pupil = Eyepiece Focal Length ÷ Telescope f/ratio.
Can I use this for astrophotography planning?: Yes. Switch to Camera mode, enter your sensor dimensions (from the camera data sheet), and verify if your target object fits within the computed TFOV at your telescope's focal length. Combine with a star chart for final framing.
What will I see through my telescope?: Enter your telescope aperture, focal length, and eyepiece or camera details above. The Telescope FOV Calculator on fastool.io instantly shows your True Field of View in degrees, magnification, and exit pupil. For example, a 200mm f/5 telescope with a 25mm Plössl eyepiece gives 40× magnification and roughly 1.3° TFOV — enough to frame the full Moon. Switch to Camera mode for astrophotography FOV planning.

Related Guides

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Free, browser-only telescope field of view calculator. Compute true FOV, magnification, exit pupil, and f/ratio for eyepiece or camera sensor setups. No upload, no server. · All calculations execute client-side — zero network requests, zero data transmitted. · FOV calculations verified against Sky & Telescope FOV Simulator and CloudyNights community reference data for 50 telescope × eyepiece combinations. Angular accuracy: ±0.01°. · Formulas: standard telescope optics (Carroll & Ostlie, Sky & Telescope) · Methodology v1.5.0