It multiplies the effective focal length of the telescope, so every eyepiece becomes two. Its quiet advantage is that eye relief survives: a comfortable long eyepiece behind a Barlow gives high magnification without pressing the eye to the glass.
A Barlow is the cheapest way to change what an eyepiece case can do, and the reason is arithmetic rather than optics.
What it multiplies
The effective focal length of the telescope. A 2x Barlow makes every eyepiece behave as though its focal length were halved, which doubles the magnification each one gives.
The advantage nobody advertises
Eye relief. Getting a high magnification by fitting a very short simple eyepiece means putting your eye almost on the glass. Getting the same figure with a longer eyepiece and a Barlow keeps the eye where it is comfortable, and comfort is what decides how long you look.
The duplication trap
A 2x Barlow with eyepieces that are already a factor of two apart reproduces figures you have. Write out the magnifications before buying: the useful case is one where the Barlow fills gaps rather than repeating values.
The feature worth looking for
Many Barlows unscrew so that the lens cell alone threads onto a camera nose. That changes the amplification and is genuinely useful for planetary imaging, where the right focal ratio has to be reached somehow.
The questions that keep arriving
Is the amplification factor fixed?
No. Moving the Barlow further from the eyepiece increases it, which happens whenever one is used ahead of a diagonal or with an extension tube. The figure on the barrel applies at the standard spacing.
Does a Barlow degrade the image?
A poor one does, on every eyepiece used behind it. A good one is close to invisible. Since it sits in the path of the whole case, it is worth more attention than a single extra eyepiece would be.
Last reviewed 17 September 2026