Telescope Field of View Calculator — FOV, Magnification & Exit Pupil
Enter your telescope and eyepiece specs to get magnification, true field of view in degrees and arcminutes, exit pupil, Dawes resolution limit and limiting stellar magnitude.
mm
mm
mm
°
78 arcmin — at 40× magnification
- 1
Magnification
1,000 ÷ 25 = 40Telescope focal length ÷ eyepiece focal length. - 2
True field of view
52° ÷ 40 = 1.300
How does this calculator work?
M = F_tel / f_ep; True FOV = AFOV / M; Exit pupil = aperture / M. A 100 mm f/10 scope (F=1000 mm) with a 25 mm / 52° AFOV eyepiece gives 40× magnification, 1.3° true FOV, 2.5 mm exit pupil and reaches magnitude ≈12. Change eyepiece focal length to trade FOV for magnification.
Formula
How this is calculated
Every telescope–eyepiece combination delivers a specific magnification and field of view determined by just four parameters. Magnification equals the telescope focal length divided by the eyepiece focal length (M = F/f). The true field of view (TFOV) is how much sky you actually see, in degrees: TFOV = AFOV / M, where AFOV is the eyepiece's apparent field of view stamped on its barrel. A 52° apparent-FOV eyepiece at 40× delivers a TFOV of 1.3° — just wide enough to frame a cluster or two.
Exit pupil is the diameter of the light beam exiting the eyepiece (exit pupil = D / M, where D is the objective aperture in mm). Ideally the exit pupil matches your eye's dark-adapted pupil (5–7 mm for adults). Exit pupils above 7 mm waste light because the excess spills onto your iris; exit pupils below 0.5 mm can make dust on the eyepiece lens distractingly visible. The Dawes limit (116 / D arcsec) is the theoretical resolving power for a perfect objective under perfect seeing — in practice atmospheric seeing often limits resolution to 1–2 arcsec.
Limiting magnitude (the faintest star visible) scales with aperture as approximately 2.1 + 5 log₁₀(D mm) — a 100 mm scope reaches about magnitude 12.1 under dark skies. Light-gathering power relative to the naked eye (7 mm pupil) is (D/7)². Maximum useful magnification is roughly 2× the aperture in mm (above this, images become unresolvably blurry for most telescopes and skies).
Frequently asked questions
An exit pupil of 5–7 mm matches the dark-adapted eye for the widest, brightest views (ideal for nebulae and Milky Way sweeping). 2–4 mm is comfortable for most deep-sky objects. Below 1 mm is reserved for high-magnification planetary work. Above 7 mm the surplus light is wasted on your iris, and the sky background also appears brighter, reducing contrast.
Apparent Field of View — the angular diameter of the "porthole" you look through. Common values are 50° (Plössl), 68° (wide-field), 82° (ultra-wide, e.g. Nagler) and up to 100° (extreme wide-angle). A wider AFOV gives a more immersive view and a larger true FOV at the same magnification.
Match the true FOV to your target's angular size. The Moon and Sun are about 0.5°; the Orion Nebula is about 1°; the Pleiades cluster is about 2°. For planets and double stars, maximise magnification up to the limit where seeing allows. A good starting set covers three magnification ranges: low (wide field, 5–7 mm exit pupil), medium (deep-sky, 2–3 mm), and high (planetary, 0.8–1.5 mm).
Also known as
TG we-Calculate Editorial Team. (2026). Telescope Field of View Calculator — FOV, Magnification & Exit Pupil [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/telescope-field-of-view-calculator
TG we-Calculate Editorial Team. "Telescope Field of View Calculator — FOV, Magnification & Exit Pupil." TG we-Calculate. 2026. https://we-calculate.com/calculator/telescope-field-of-view-calculator.
TG we-Calculate Editorial Team, "Telescope Field of View Calculator — FOV, Magnification & Exit Pupil," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/telescope-field-of-view-calculator
@misc{wecalculate_telescope_field_of_view_calculator, title = {Telescope Field of View Calculator — FOV, Magnification & Exit Pupil}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/telescope-field-of-view-calculator}}, year = {2026}, note = {TG we-Calculate} }
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