Barlow lens

The practical result is that inserting a Barlow lens magnifies the image.

[1] 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.

Astronomical Barlow lenses are rated for the amount of magnification they induce.

A common misconception is that higher magnification always equates to a higher-quality image.

In microscopy the Barlow lens is used to increase working distance and decrease magnification.

Cone of light behind an achromatic doublet objective lens (A) without (red) and with (green) a Barlow lens optical element (B)
A Boeing 747-400 aircraft at 11,000 metres, photographed from the ground using a 1,200 mm telescope and a 2x Barlow lens.