Telescope Field of View Calculator
Calculate the true field of view for your telescope and eyepiece combination. Enter values for instant results with step-by-step formulas.
Formula
True FOV = Apparent FOV / Magnification; Magnification = Scope FL / Eyepiece FL
True field of view divides the eyepiece apparent field of view by the magnification. Magnification is the telescope focal length divided by the eyepiece focal length, multiplied by any Barlow factor. Exit pupil equals the aperture divided by magnification.
Worked Examples
Example 1: Newtonian Reflector with Standard Eyepiece
Problem: An 8-inch (200mm) f/6 Newtonian with 1200mm focal length uses a 25mm Plossl eyepiece (52 degrees AFOV), no Barlow.
Solution: Magnification = 1200 / 25 = 48x\nTrue FOV = 52 / 48 = 1.083 degrees = 65.0 arc minutes\nExit pupil = 200 / 48 = 4.17mm\nf-ratio = 1200 / 200 = f/6\nMax useful magnification = 200 x 2 = 400x\nMoon diameters in FOV = 1.083 / 0.5 = 2.17\nDawes limit = 116 / 200 = 0.58 arcseconds
Result: True FOV: 1.083 degrees | Magnification: 48x | Exit Pupil: 4.17mm
Example 2: SCT with Barlow for Planetary Viewing
Problem: A 203mm (8-inch) SCT with 2032mm focal length uses a 10mm eyepiece (60 degrees AFOV) with a 2x Barlow lens.
Solution: Effective focal length = 2032 x 2 = 4064mm\nMagnification = 4064 / 10 = 406.4x\nTrue FOV = 60 / 406.4 = 0.148 degrees = 8.9 arc minutes\nExit pupil = 203 / 406.4 = 0.50mm\nMax useful mag = 203 x 2 = 406x\nMoon diameters = 0.148 / 0.5 = 0.30\nDawes limit = 116 / 203 = 0.57 arcseconds
Result: True FOV: 0.148 degrees | Magnification: 406x | Exit Pupil: 0.50mm
Frequently Asked Questions
What is field of view and why does it matter for telescopes?
Field of view is the angular extent of the sky visible through the telescope and eyepiece combination at any given moment. True field of view (TFOV) measures the actual sky area you can see, expressed in degrees. A wider field of view shows more sky area, which is essential for locating objects, observing extended objects like nebulae and star clusters, and providing context around your target. Narrow fields of view are better for resolving fine details on planets and double stars. Understanding your field of view helps you plan observations, estimate the angular size of objects, and select the right eyepiece for each target. For example, the Andromeda Galaxy spans about three degrees, requiring a very wide field to see it entirely.
How do you calculate true field of view from apparent field of view?
True field of view is calculated by dividing the eyepiece apparent field of view by the magnification. The apparent field of view is a fixed property of the eyepiece design, typically ranging from forty degrees for basic Kellner designs to eighty-two degrees or more for premium wide-angle eyepieces. Magnification is the telescope focal length divided by the eyepiece focal length. For example, a telescope with 1200mm focal length using a 25mm eyepiece with 52 degrees apparent field: magnification equals 1200 divided by 25 equals 48x, and true field equals 52 divided by 48 equals 1.08 degrees or about 65 arc minutes. This means you see a circle of sky roughly two full moon diameters across.
How does a Barlow lens affect field of view and magnification?
A Barlow lens is a diverging lens placed before the eyepiece that increases the effective focal length of the telescope, thereby multiplying the magnification by its power factor, typically two or three times. Since true field of view equals apparent field of view divided by magnification, doubling the magnification with a two-times Barlow halves the true field of view. A Barlow effectively doubles your eyepiece collection because each eyepiece can produce two different magnifications. The advantage is achieving higher magnification while maintaining the eye relief of the original eyepiece, which is particularly beneficial for eyeglass wearers. Quality Barlows introduce minimal optical degradation, but cheap ones can reduce sharpness and introduce chromatic aberration.
What formula does Telescope Field of View Calculator use?
The formula used is described in the Formula section on this page. It is based on widely accepted standards in the relevant field. If you need a specific reference or citation, the References section provides links to authoritative sources.
Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.