The eyepiece is basically a projection lens system. The most common type is the Huygenian . This eyepiece is used with low to medium magnifications, and is designed to project the image into a human eye. These provide superior image quality. Another important type of eyepieces is the photo eyepiece - designed to project a corrected image onto a camera's film plane.
All eyepieces have a relative magnification - written on the side of the barrel.
They range in magnification from 2.5 X to 15 X - with the lower magnifications used for the photo eyepiece.