Telescope Field of View Calculator
Calculate the true and apparent field of view for your telescope and eyepiece combination.
Field of view (FOV) describes how much sky you can see through your telescope at one time. A wider field of view lets you see larger areas of the sky, making it easier to find objects and enjoy wide vistas like star clusters and nebulae. A narrower field gives higher magnification for detailed views of planets and the Moon.
Key formulas:
Magnification:
Magnification = Telescope Focal Length / Eyepiece Focal Length
A telescope with a 1200mm focal length using a 25mm eyepiece gives 48x magnification.
True Field of View (TFOV):
TFOV = Apparent FOV of Eyepiece / Magnification
The true field of view tells you the actual angular extent of sky visible through the telescope, measured in degrees.
Apparent Field of View (AFOV): This is a property of the eyepiece design, printed on the eyepiece barrel:
- Standard (Kellner, Plossl): 50-52° AFOV
- Wide-angle: 60-68° AFOV
- Ultra-wide: 82-100° AFOV
Linear field of view:
To calculate how wide an area you see at a specific distance:
Linear FOV = 2 × distance × tan(TFOV / 2)
For astronomical purposes, the Moon is approximately 0.5° in angular diameter. If your TFOV is 1°, you will see about two Moon-widths of sky.
Maximum useful magnification:
Every telescope has a practical maximum magnification limited by its aperture:
Max Useful Magnification ≈ 2 × Aperture in mm
Beyond this, images become dim and blurry due to diffraction limits. A 200mm telescope has a useful maximum of about 400x.
Exit pupil:
Exit Pupil = Eyepiece Focal Length / Telescope f-ratio
f-ratio = Telescope Focal Length / Aperture
The exit pupil should not exceed your eye’s pupil diameter (about 7mm for young adults in darkness, decreasing with age). An exit pupil of 2-4mm is ideal for most deep-sky observing.
Choosing eyepieces for different targets:
- Wide TFOV (1-3°): Best for finding objects, viewing large nebulae and star clusters
- Medium TFOV (0.3-1°): Good general purpose, nice views of galaxies and smaller nebulae
- Narrow TFOV (under 0.3°): Best for planets, double stars, and lunar detail
Barlow lenses effectively double (2x) or triple (3x) the magnification of any eyepiece by increasing the effective focal length. They are an economical way to increase your eyepiece collection.