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How does an eyepiece change a telescope’s magnification?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

In a telescope, the eyepiece acts as a secondary lens system placed behind the focal plane, allowing an observer to view the image formed by the telescope's objective.1

The eyepiece determines the total magnification of the instrument. You can calculate the magnification by dividing the focal length of the objective by the focal length of the eyepiece. For example, if a telescope has an objective focal length of 254 cm and you use an eyepiece with a focal length of 2.54 cm, the resulting magnification is 100.1

By changing the eyepiece to one with a different focal length, an observer can adjust the magnification to suit their needs, such as using higher magnification for detailed views of the Moon or planets. However, it is important to note that practical magnification is ultimately limited by the telescope's resolving power, and high magnification requires a very stable mounting to prevent vibrations from degrading the image quality.12