Barlow lenses are used to multiply the focal length of the objective lens/mirror of your
telescope and therefore its magnifying power. The benefits of using a Barlow lens will
result in image magnification, improved eye relief and reduced spherical aberration in
Barlow lenses can also be used with binoviewers that in certain cases are unable to
reach focus, especially when used with Newtonian telescopes and refractors with short focus
travel. The extended focal length provided by the Barlow will then make focusing possible.
Sometimes binoviewers are supplied with their own detachable barlow lens.