Magnification (Power) And Using Eyepieces . ... To determine power in a telescope, divide the focal length of the telescope by the focal length of the eyepiece. By exchanging an eyepiece of one focal length for another, you can increase or decrease the power of the telescope. For example, a 25mm eyepiece used on a telescope with a 1000mm focal length would yield a power of 40x (1000 / 25 = 40) and a 10mm eyepiece used on the same telescope would yield a power of 100x (1000 / 10 = 100). ...