Telescope Field of View Eyepiece Magnification Degrees

True field of view from eyepiece and magnification

The true field of view (TFOV) is how much sky you actually see through the eyepiece, in degrees. To get it, take the eyepiece’s apparent field of view (AFOV) and divide by the magnification: TFOV = AFOV / magnification. So a 50° eyepiece at 100× works out to 50 / 100 = 0.5°, roughly the width of the full Moon.

AFOV is baked into the eyepiece design and doesn’t change (usually 50° on Plössls, somewhere around 68–82° on wide-field models). Magnification, meanwhile, is just the telescope’s focal length over the eyepiece focal length. Crank up the magnification and the true field always shrinks. That’s the reason observers reach for low-power, wide-AFOV eyepieces when chasing large objects or hunting for a target.

Applications

The TFOV tells you whether an extended object will actually fit in the view — the Moon (0.5°), the Pleiades (about 2°), or the Andromeda Galaxy (over 3°). It also helps you map out star-hopping routes and work out how long a star takes to drift across the field at a given magnification.

FAQ

Where do I find the AFOV? The manufacturer lists it in the eyepiece specs (e.g. 52°, 68°, 82°).

Why is my real field a bit smaller? This quick formula skips the field-stop diameter and edge distortion. A precise TFOV is computed from the eyepiece field stop, but the plain division still lands within a few percent.

How do I get a wider field? Drop to a lower magnification with a longer eyepiece focal length, or switch to an eyepiece with a bigger apparent field of view.