Barlow lenses are concave or negative lenses that amplify or magnify the image produced by your telescope. They work by lengthening the converging cone of light from the scope, effectively increasing its focal length.

Does a Barlow lens double the focal length?

A Barlow Lens, however, increases the magnification by effectively increasing the focal length of the telescope instead. Thus, giving us the ability to use a wider field of view for lenses to achieve the same magnification. A 2X Barlow lens doubles your telescope's focal length while a 3X Barlow lens triples it.

How does a Barlow affect field of view?

A barlow lens increases magnification by some factor. A 2X barlow will double the magnification of any given eyepiece. But it does not affect that apparent field of view of the eyepiece, the AFOV.