field of view

The field of view (often abbreviated as FOV) is the amount of sky you can see at one time when looking through a telescope or binoculars, measured in degrees. The actual field of view is usually small. For example, if your telescope just shows the whole of the full Moon completely filling the view, its actual field of view will be about half a degree, which is the diameter of the Moon in the sky. However, there’s also the apparent field of view. Some eyepieces have a small apparent field of view, maybe 30º, which makes it look as if you are viewing the Moon through a tube. Other eyepieces have a much wider field of view, say 60º or even 80º, so the view is spread out over a much wider area.

Field of view of three eyepieces

How the Moon would look through three different eyepieces.