Glossary | Telescope accessories | Capacity | Apparent field of view

The greater the apparent field of view, the greater the true field of view of the sky that can be seen.

The true field of view is derived from the apparent field of view of the eyepiece and the magnification, using the following formula:

True field of view = apparent field of view / magnification

Magnification, in turn, is derived from the ratio of the focal length of the telescope to that of the eyepiece.

The size of the visible area of the sky therefore depends on both the apparent field of view and the focal length of the eyepiece as well as the focal length of the telescope.

Example: a Super Plössl eyepiece with a 52° apparent field of view and 10 millimeter focal length combined with a telescope with a focal length of 1000 millimeters, leads to a 100x magnification and a field of view of 0.52°. This is roughly equivalent to the angular diameter of the Moon. With this combination of telescope and eyepiece it can be observed in its entirety.

An eyepiece with large apparent field of view and a high focal length displays a large section of the sky. If an eyepiece with a shorter focal length is selected, although the magnification is increased, the area of visible sky is smaller. Therefore, when observing, it is best to start with a long focal length eyepiece for an overview, and as preparation for higher levels of magnification.