Barlow lenses are concave lenses used for increasing magnification power of a telescope eyepiece. It is easy to calculate the resulting magnification: multiply the magnification of your telescope eyepiece by the magnification of a Barlow lens. For instance, let us take a telescope eyepiece that provides 25x magnification. When we use a 2x Barlow lens, the magnification increases to 50x. These accessories can be used for astrophotography as well as for observations.