Telescope Field of View Calculator
Find how much sky you will see through your telescope. Enter the telescope focal length with an eyepiece (apparent field or field stop) or a camera sensor to get the true field of view, magnification and image scale, then check it against a to-scale full Moon and popular deep-sky targets.
Last updated:
for exit pupil and focal ratio
Plössl ≈ 50–52°, wide-angle 68–82°, ultra-wide 100°
from the eyepiece specs; most accurate method
for image scale in arcseconds per pixel
How to use the Telescope Field of View Calculator
- Choose a method: eyepiece with its apparent field, eyepiece field stop, or camera sensor.
- Enter the telescope focal length and any Barlow or reducer you use.
- Enter the eyepiece or sensor details; they are printed on the eyepiece or in the camera specs.
- Press Calculate to see the true field in degrees and arcminutes.
- Check the overlay and the fit table to see whether your target fits in the view.
Field of view formulas
- Apparent field method:
True FOV = AFOV ÷ (Ftelescope ÷ Feyepiece). Quick and good to about 5–10%. - Field stop method:
True FOV (°) = field stop (mm) ÷ Ftelescope (mm) × 57.296. The most accurate for eyepieces. - Camera sensor:
FOV = 2 × atan(sensor dimension ÷ (2 × Ftelescope))for width, height and diagonal. - Image scale:
arcsec per pixel = 206.265 × pixel size (µm) ÷ F (mm).
Multiply the telescope focal length by your Barlow or reducer factor first. One degree is 60 arcminutes (′) and one arcminute is 60 arcseconds (″).
Typical true fields (1,200 mm telescope)
| Eyepiece | Apparent field | Magnification | True field | Good for |
|---|---|---|---|---|
| 32 mm Plössl | 50° | 38× | 1.33° (80′) | Whole Moon, big star clusters |
| 25 mm Plössl | 52° | 48× | 1.08° (65′) | Moon, Orion Nebula, finding targets |
| 16 mm wide-angle | 82° | 75× | 1.09° (66′) | Globular clusters with a wide view |
| 10 mm | 52° | 120× | 0.43° (26′) | Lunar close-ups, small nebulae |
| 6 mm | 60° | 200× | 0.30° (18′) | Planets, double stars |
Tips for picking an eyepiece
Use your lowest-power, widest-field eyepiece to find objects, then switch up. Large objects such as the Pleiades (about 2°) and the Andromeda Galaxy (over 3° long) need a short focal length telescope or a 2″ eyepiece to fit. Planets are tiny (under 1′), so field of view matters less than magnification and steady air. Without a tracking mount, objects drift across the field in a few minutes at high power, which is why a wide apparent field makes high magnification easier to use.
Frequently asked questions
How do you calculate a telescope's true field of view?
Divide the eyepiece's apparent field of view by the magnification, where magnification = telescope focal length ÷ eyepiece focal length. A 25 mm, 52° eyepiece in a 1,200 mm telescope gives 48× and a true field of 52 ÷ 48 ≈ 1.08°, about 65 arcminutes.
What is the field stop method?
The field stop is the ring inside the eyepiece barrel that sharply bounds the view. True field (degrees) = field stop diameter ÷ telescope focal length × 57.3. It is more accurate than the apparent-field method because AFOV figures are rounded and affected by distortion.
What is the difference between apparent and true field of view?
Apparent field is how wide the view looks to your eye through the eyepiece alone, such as 52° or 82°. True field is the actual patch of sky you see, which is the apparent field shrunk by the magnification.
Will the full Moon fit in my eyepiece?
The full Moon is about 0.5° (29.3′ to 34.1′ depending on its distance). It fits whenever the true field is larger than about 0.55°. Low-power eyepieces with long focal lengths and wide apparent fields show the whole Moon with space around it.
How do I find the field of view for a camera on a telescope?
For each side of the sensor, FOV = 2 × atan(sensor size ÷ (2 × focal length)). An APS-C sensor (23.5 × 15.6 mm) at 400 mm covers about 3.4° × 2.2°. Image scale is 206.265 × pixel size (µm) ÷ focal length (mm) arcseconds per pixel.
What is the widest field my telescope can show?
It is limited by the largest field stop that fits through the focuser: about 27 mm for 1.25″ eyepieces and about 46 mm for 2″ eyepieces. Divide that by the focal length and multiply by 57.3 to get the maximum true field in degrees.
Does a Barlow lens change the field of view?
Yes. A 2× Barlow doubles the effective focal length, which doubles magnification and halves the true field. A 0.63× focal reducer does the opposite and widens the field by about 1.6×.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.