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In its Astronomical use, a Barlow lens may be placed immediately before an Eyepiece to effectively decrease the eyepiece's Focal Length by the amount of the Barlow's divergence. Since the Magnification provided by a Telescope and eyepiece is equal to the telescope's focal length divided by the eyepiece's focal length, this has the effect of increasing the magnification of the image. For this reason, astronomical Barlow lenses are named for the amount of magnification they induce. Most commonly, Barlow lenses are 2x, but 3x Barlows and adjustable Barlows are available.

Barlow lenses have an unfortunate reputation in some quarters due to poor results given by the Barlows included with some department-store telescopes. These results, however, are perhaps better attributed to poor optics and shaky Mounts permitting less practical magnification in these telescopes than to an inherent failing of Barlow lenses.


ADVANTAGES AND DISADVANTAGES


Assuming that the Barlow is a good one, the only disadvantage is a slight loss of light throughput – this is of the order of 3%. The advantages are numerous:

Higher magnifications can be attained with longer focal-length eyepieces than would be possible without the Barlow. Short focal length eyepieces necessarily have optical surfaces that are more curved and therefore are likely to introduce more aberrations.
A Barlow increases the effective focal ratio of the objective. This gives a more acute light cone, which is less demanding of eyepiece quality because:
Rays at the periphery of the cone are closer to being paraxial and thus are less subject to aberration.
A smaller area of the field lens is used.
Many eyepieces have an eye relief (distance of exit pupil from eye lens) that is directly related to its focal length. For example, the eye relief of a Plössl is 0.73 × its focal length. Thus, with these eyepieces, for a given magnification there will be greater eye relief with a barlow than without.
Many eyepiece types do not work well with short focal-ratio objectives. The Barlow effectively increases the focal ratio, allowing the eyepiece to work well, even at shorter focal lengths.