Field of view, as the term suggests, relates to the amount of sky visible through the eyepiece. This again depends upon the focal length of the telescope and one other characteristic, the size of the aperture. And the focal length of the eyepiece. As the magnification is increased the field of view is decreased. That is why when viewing deep sky objects, we normally use low magnification and therefore a wider field of view.

The field of view is mostly governed by the size and type of eyepiece. But the aperture of the telescope will also play a small part. Generally a larger aperture telescope will give a larger field of view. To calculate the field of view or FOV we must first know the FOV of the eyepiece we are using. This information is normally supplied with the eyepiece, along with its focal length and is given in degrees. For this exercise I will assume your eyepiece has a quoted FOV of 50 degrees and a focal length of 25mm.

FOV = magnification of telescope and eyepiece set-up divided by the FOV of the eyepiece.

For example our 500mm focal length telescope when used with a 25mm eyepiece, will provide a magnification of 20 times. The eyepiece has a quoted field of view (FOV) of 50 degrees. So that is 20 divided by 50 which gives us a FOV of 0.4 of a degree.

This means our telescope when used with this eyepiece will let us view about half of a degree of the night sky. That is about as much sky as is occupied by the full Moon. So the Moon would just about fill the whole of the view through this 25mm eyepiece.