The Barlow lens is used to multiply the focal length (Magnifying Power) of the objective lens. It is inserted between the diagonal & the eyepiece. The benefits of using a Barlow lens to increase magnification are: 1. enhanced eye relief. 2. Reduced spherical Aberration in the eyepiece. This Deluxe version of the Sky-Watcher Barlow uses two lens elements, as opposed to a single element in the economy version.