Field of View Calculator

What the Camera Field of View Calculator Does

This calculator tells you how much of the sky your camera and telescope (or lens) will capture in a single frame. You enter your sensor's physical dimensions and the focal length of the optical system, and it returns the field of view (FOV) in degrees, usually broken out into width and height, and converted to arcminutes for finer work.

It is built for astrophotographers and visual observers who need to know whether a deep-sky target will actually fit in frame. If you are deciding between two telescopes, choosing a camera, or planning a mosaic, this is the tool that confirms framing before you spend a clear night on it.

How It Works: The Field of View Formula

The field of view depends on the angle subtended by the sensor as seen through the optics. For each dimension of the sensor, the formula is:

FOV (degrees) = 2 × atan( sensor dimension / (2 × focal length) ) × (180 / pi)

Sensor dimension and focal length must use the same units (both in millimeters). Because a sensor has a width and a height, you run the formula twice to get the horizontal and vertical FOV. To express the result in arcminutes, multiply the degrees by 60 (and by 3600 for arcseconds).

• Sensor dimension: the physical width or height of the imaging chip in mm, not the megapixel count.
• Focal length: the telescope or lens focal length in mm, after any focal reducer or Barlow is applied.
• atan: the arctangent function; the factor of 2 accounts for the angle on both sides of the optical axis.

Worked Example

Suppose you pair an APS-C camera with a sensor measuring 23.5 mm × 15.6 mm with a telescope of 600 mm focal length.

Horizontal: FOV = 2 × atan(23.5 / (2 × 600)) = 2 × atan(0.01958) = 2 × 1.1217 deg = 2.243 degrees, which is about 134.6 arcminutes.

Vertical: FOV = 2 × atan(15.6 / (2 × 600)) = 2 × atan(0.01300) = 2 × 0.7448 deg = 1.490 degrees, or about 89.4 arcminutes. So this rig frames roughly 2.24 by 1.49 degrees. The Andromeda Galaxy spans about 3 degrees, so it would not fit in one frame, while the Orion Nebula (about 1 degree across) sits comfortably inside.

Tips and Common Mistakes

Small errors in your inputs produce large framing surprises, so check these before trusting the result.

• Use the true focal length: a 0.8x reducer turns a 600 mm scope into 480 mm and widens the FOV; a 2x Barlow doubles focal length and narrows it.
• Enter sensor size, not megapixels: two cameras with identical pixel counts can have very different chip sizes.
• Do not confuse focal length with aperture: aperture (and f-ratio) affect brightness and exposure, but FOV depends only on sensor size and focal length.
• For small angles the linear shortcut FOV (arcmin) approx 3438 × dimension / focal length is close, but the atan formula stays accurate even at wide fields and short focal lengths.

Factors That Affect the Result

The largest driver of FOV is the ratio between sensor size and focal length. A larger sensor or a shorter focal length both widen the field; a longer focal length narrows it for a higher-magnification, tighter crop.

A few practical factors shift the usable field beyond the raw math. Vignetting and optical aberrations can darken or soften the corners, so the clean, frame-worthy area may be slightly smaller than the calculated FOV. Image circle also matters: if your sensor is larger than the corrected image circle of the telescope, the corners may be unusable even though the geometric FOV is large.

When a target is wider than your computed field, plan a mosaic by overlapping adjacent frames (commonly 15 to 25 percent overlap) so they stitch cleanly.