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Barlow lens

Magnification of telescopes is calculated by dividing the objective focal length with the eyepiece focal length. To change the magnification, a different eyepiece must be inserted into the telescope. Another way to change the magnification is the use of a Barlow lens. A Barlow lens increases the effective focal length of the telescope, which means that magnification increases for any eyepiece placed after the Barlow.

Barlow lens
The path of the light, focusing at point A – without the Barlow. The path of the light using the Barlow- focus point is moved to point B (Source:

Barlow is a concave lens that contains several glass elements. Because of that, it can reduce chromatic aberration.

Depending on the type of Barlow, the magnification can be doubled (2x) or tripled (3x), but there are also other Barlows available.

Using a Barlow lens is a very cost-effective way to change the magnification. It can be used with all the telescope’s eyepieces. To use the Barlow, the eyepiece needs to be removed from the focuser and then replaced with the Barlow lens. The eyepiece is then connected to the Barlow.



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