About the Telescope Field of View Calculator
This telescope field of view calculator tells you how big a patch of sky you will see through a given eyepiece or capture on a camera sensor. Enter your telescope’s focal length and aperture, then either the eyepiece focal length and apparent field of view, the eyepiece field-stop diameter, or your camera sensor size — and it returns the true field of view in degrees and arcminutes.
Visual observers use it to choose an eyepiece that frames a target — for example whether the whole Pleiades cluster or Andromeda galaxy fits — while astrophotographers use the sensor mode to plan framing and check image scale in arcseconds per pixel. The result is also expressed in full-Moon widths, which is an intuitive way to picture it.
The apparent-FOV method (true field = apparent field ÷ magnification) is a close approximation; the field-stop method is more accurate when the manufacturer publishes the field stop. Barlow lenses and focal reducers change the effective focal length, so multiply or divide the focal length before entering it.
With the default inputs, the true field of view is 1.083 °. Change any value above to recalculate instantly.
How to use the telescope field of view calculator
- 1Choose the method: apparent field, field stop, or camera sensor.
- 2Enter the telescope focal length and aperture in millimetres.
- 3Enter the eyepiece focal length and apparent field (or its field stop), or your sensor size and pixel size.
- 4Read the true field of view in degrees, arcminutes and full-Moon widths.
- 5Check magnification and exit pupil, or image scale for imaging.
Formula and method
Magnification is the telescope focal length divided by the eyepiece focal length. The quick method divides the eyepiece’s apparent field of view (the angle you see the field circle span) by the magnification. The more accurate method uses the eyepiece field stop — the physical aperture that limits the view — divided by the telescope focal length, converted from radians to degrees.
For a camera the sensor sits at the focal plane, so each sensor dimension subtends 2·atan(size ÷ 2F). Image scale is 206.265 × pixel size (µm) ÷ focal length (mm) arcseconds per pixel. Exit pupil is aperture ÷ magnification; full-Moon widths assume a 0.52° Moon.
- F
- Telescope focal length (mm)
- f
- Eyepiece focal length (mm)
- AFOV
- Eyepiece apparent field of view (degrees)
- TFOV
- True field of view on the sky (degrees)
- w
- Sensor width or height (mm)
Worked examples
8-inch f/6 Dobsonian with a 25 mm Plössl
1200 mm ÷ 25 mm = 48×. A 52° apparent field ÷ 48 gives a true field of 1.08° (65 arcminutes) — about two full Moons — with a comfortable 4.2 mm exit pupil.
Same scope, eyepiece with a 27 mm field stop
27 ÷ 1200 = 0.0225 radians × 57.296 = 1.289°, or 77.3 arcminutes. The field-stop method is exact for the eyepiece regardless of its quoted apparent field.
APS-C camera (23.5 × 15.6 mm, 3.76 µm pixels) at 1200 mm
Width: 2·atan(23.5 ÷ 2400) = 1.122°; height 0.745°; diagonal 1.347°. Image scale is 206.265 × 3.76 ÷ 1200 ≈ 0.65″ per pixel — fine sampling suited to good seeing.
80 mm f/6 refractor with a 32 mm, 50° eyepiece
480 ÷ 32 = 15×, so the true field is 50 ÷ 15 = 3.33° — wide enough for the whole Pleiades or Andromeda galaxy — with a 5.3 mm exit pupil.
Frequently asked questions
What is the difference between apparent and true field of view?+
Apparent field is the angle the eyepiece’s view circle appears to span to your eye, set by the eyepiece design (typically 50–100°). True field is the actual patch of sky shown, equal to the apparent field divided by the magnification.
How do I get a wider field of view?+
Use a longer focal length eyepiece, an eyepiece with a wider apparent field, or a telescope with a shorter focal length. A 2-inch eyepiece barrel allows a larger field stop and therefore the widest possible true field.
What is the field stop of an eyepiece?+
It is the ring inside the eyepiece barrel that sharply bounds the view. Its diameter divided by the telescope focal length (in radians) gives the true field exactly, so many observers prefer it to the apparent-field estimate.
How does a Barlow lens or focal reducer change the field of view?+
A 2× Barlow doubles the effective focal length, halving the true field and doubling magnification. A 0.8× reducer shortens focal length, widening the field by 25%. Enter the effective focal length to model either.
What image scale should I aim for in astrophotography?+
Under typical 2–3 arcsecond seeing, roughly 1–2 arcseconds per pixel is a good match. Much smaller values oversample (no extra detail, less signal per pixel), while much larger values undersample and look blocky.