The eyepiece you look through in a telescope helps you view the focused image gathered by the telescope's mechanism of lenses and mirrors. Eyepieces carry different sizes and magnifications, which are used according to the distance and detail of the object you're viewing.

Refracting Telescopes

In a refracting telescope, the eyepiece focuses light that strikes the objective (front) lens directly. This is the simplest and oldest telescope design, and is also the principle on which binoculars and telephoto camera lenses work.

Reflecting Telescope

In a reflecting telescope, the light from the object strikes a series of mirrors within the barrel of the telescope. The eyepiece is fixed on the mirror rather than directed toward the object. There are many different configurations for reflecting telescopes; one of the most common is to have the lens fixed into the side of the telescope with the viewer looking downward at the image reflected from the objective mirror.

Focal Length

Eyepieces vary by focal length, ranging from 2 to 60 millimeters. The focal length of the telescope, divided by the focal length of the eyepiece, determines magnification. An eyepiece with a focal length of 20 millimeters, used in a telescope with a focal length of 800 millimeters, provides 40x ("40-power") magnification.

Barrel Length and Field of View

Eyepieces also vary by diameter, with 2 inches, 1.25 inches, and 24.5 millimeters the standard sizes. They are also measured by field of view, which is the measurement of the width of the image, in degrees, from one edge to the other. This varies from 30 to 80 degrees.

The Power of Low Magnification

Low-magnification eyepieces, which give the user a wider field of view and focus more light, are used to more easily view the most distant objects. High-magnification lenses are more useful for closer objects such as the moon and planets. The high-magnification lenses are more difficult to use on distant objects, as they are affected by disturbances in the atmosphere, an unsteady telescope and flaws in the optics within the telescope.

Source:

Astronomical Telescope Eyepieces: A Guide for Beginners

Telescopes: Eyepieces

Optics Planet: Ten Questions About Telescope Eyepieces