Telescope field of view calculator for cameras
Enter a camera and a telescope (or pick them from the lists) to get the field width, height and diagonal in degrees and arcminutes, the image scale and the focal ratio. The calculator uses the exact formula 2 × arctan(sensor size ÷ 2 × focal length) and draws the frame next to the Moon for scale.
How to use it
- Pick your camera, or choose “Custom sensor” and type the sensor width and height in millimetres. For a DSLR or mirrorless body, the maker’s spec page gives the sensor size; the camera tables list the values we found.
- Pick your telescope or type the focal length. If the telescope has an official reducer or flattener in our data, it appears in the second list; otherwise type its factor (0.8 for a 0.8× reducer, 2 for a 2× Barlow).
- Read the field width and height. The chart draws your frame to scale with the Moon in the corner, north up and east left as on a star atlas.
The aperture is only needed for the focal ratio, and the pixel size only for the image scale. The field of view itself depends on two numbers: the size of the sensor and the effective focal length.
The formula
Each side of the sensor sees an angle set by simple trigonometry: half the side, divided by the focal length, is the tangent of half the angle.
field angle = 2 × arctan( sensor side ÷ (2 × focal length) )
Many calculators use the shortcut angle (radians) ≈ side ÷ focal length. At telescope focal lengths the two agree closely, but the shortcut overstates the field more and more as the focal length gets shorter. For the 36 mm width of a full-frame sensor:
| Focal length | Exact width | Shortcut | Shortcut error |
|---|---|---|---|
| 14 mm | 104.25° | 147.33° | +41 % |
| 24 mm | 73.74° | 85.94° | +17 % |
| 50 mm | 39.60° | 41.25° | +4.2 % |
| 135 mm | 15.19° | 15.28° | +0.6 % |
| 400 mm | 5.15° | 5.16° | +0.07 % |
The exact form also gives the right answer for camera lenses used for Milky Way and constellation work, as long as the lens is rectilinear. Fisheye lenses map angles differently and are outside what this formula describes. The field of view guide goes through the geometry step by step.
Which sensor size the calculator uses
Camera makers describe sensor size in several ways: an inch-type label (“1-inch”, “4/3-inch”), a format name (“APS-C”), millimetre dimensions, or only the pixel count and pixel size. The field of view needs the dimensions of the area that records light. When a camera in the list has both a published pixel count and a published pixel size, the calculator multiplies them; for the ZWO ASI2600MC Pro that is 6248 × 3.76 µm = 23.49 mm by 4176 × 3.76 µm = 15.70 mm, against the “23.5 × 15.7 mm” on the spec page. When only millimetre dimensions are published, it uses those. The camera tables mark which values are computed.
Worked example
An ASI2600MC Pro (23.49 × 15.70 mm) on a Sky-Watcher Evostar 80EDX (600 mm):
- width: 2 × arctan(23.49 ÷ 1200) = 2.24°
- height: 2 × arctan(15.70 ÷ 1200) = 1.50°
- diagonal: 2 × arctan(28.25 ÷ 1200) = 2.70°
- image scale: 206.265 × 3.76 ÷ 600 = 1.29″ per pixel
The field is about 4.3 Moon diameters wide, using the Moon’s mean apparent diameter of 1896″ (31.6′) from NASA’s Moon Fact Sheet. With the official 0.85× reducer (510 mm), the field grows to 2.64° × 1.76° and the scale to 1.52″ per pixel.
What the result does not tell you
- Whether the corners are usable. A telescope’s image circle (published for many astrographs in the telescope tables) must cover the sensor diagonal, or the corners will be dark or distorted.
- Whether a target fits. Use the target framing calculator, which compares the field with catalogue sizes and position angles.
- Whether the pixels match the sky. The image scale only means something next to the seeing; the sampling calculator makes that comparison.
Questions
- How do I calculate the field of view of a telescope and camera?
- For each side of the sensor, field angle = 2 × arctan(sensor side ÷ (2 × focal length)). An APS-C sensor 23.49 mm wide on a 600 mm telescope covers 2 × arctan(23.49 ÷ 1200) = 2.24°.
- Why does the calculator use pixel count × pixel size instead of the published sensor size?
- Pixel count × pixel size is the area that actually records the image. Published sensor sizes are often rounded or describe a slightly larger package, so where both pixel values are published the calculator uses them.
- Does a focal reducer change the field of view?
- Yes. A reducer shortens the effective focal length by its factor, so the field grows by roughly the inverse: a 0.85× reducer on a 600 mm telescope gives 510 mm and a field about 18 % wider.
Sources
- Moon Fact Sheet NASA Space Science Data Coordinated Archive. Apparent diameter, mean value at opposition: 1896 arcseconds. Accessed October 12, 2026.