Field of View Calculator
Pick a camera sensor or an eyepiece, enter your telescope's focal length and see how much sky fits in the frame, measured against the full Moon.
Field of view
How much sky a camera sensor or an eyepiece takes in.
1 for none, 0.8 for a reducer, 2 for a 2x Barlow.
Field of view
3° 26.2′ x 2° 17.5′
206.2′ x 137.5′
- Width
- 3.437°
- Height
- 2.292°
- Diagonal
- 4° 7.8′
- Sky area
- 7.874 sq deg
About 6.6 full Moons fit side by side across the field.
Uses the exact angle 2 × atan(size ÷ (2 × focal length)), so it holds for wide camera lenses too.
How this is calculated
- 1
Camera field
Each side of the field is 2 × atan(sensor size ÷ (2 × focal length)). A full frame sensor (36 x 24 mm) at 600 mm covers about 3.44° x 2.29°. The diagonal uses the sensor's diagonal in the same formula.
- 2
Barlows and reducers
Multiply the focal length by the factor first. A 0.8x reducer turns 600 mm into 480 mm and widens the field by about a quarter; a 2x Barlow halves it.
- 3
Eyepiece field
Magnification is telescope focal length ÷ eyepiece focal length. The true field is roughly the apparent field ÷ magnification, so a 52° eyepiece at 24x shows about 2.17°.
- 4
Field stop method
If you know the eyepiece's field stop diameter, the true field is about field stop ÷ telescope focal length × 57.3°. This is more accurate because it does not depend on the quoted apparent field.
Frequently asked questions
How do I calculate a telescope's field of view?
For a camera, use 2 × atan(sensor width ÷ (2 × focal length)) for the width, and the same with the height. For an eyepiece, divide its apparent field by the magnification, or for better accuracy divide the field stop by the telescope's focal length and multiply by 57.3.
How big is the Moon in a telescope's field of view?
The full Moon is about 0.52° or 31 arcminutes across. If your field is 1° wide, the Moon fills about half of it. Fields narrower than 0.5° show only part of the Moon.
Does a smaller sensor magnify the image?
No. The image the telescope forms stays the same size, and a smaller sensor simply crops a smaller part of it. That is why small sensors are said to have a crop factor.
What is the difference between apparent and true field of view?
The apparent field is how wide the view looks when you put your eye to the eyepiece, set by its design. The true field is the actual patch of sky you see, which shrinks as magnification rises.