Determining magnification: The power of an eyepiece is found by dividing the focal length of your telescope by the focal length of the eyepiece. For example,a 20mm eyepiece on a 2000mm telescope (2000/20) gives you 100 power (100x). This makes objects appear 100 times closer to you through the telescope than they appear to your unaided eye. The same 20mm eyepiece on a 1000mm focal length scope (1000/20) gives you 50x. The longer the telescope focal length, or the shorter the eyepiece focal length,the higher the power - but the dimmer the image.