The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens which, used in series with other optics in an optical system, increases the effective focal ratio of an optical system as perceived by all components after it in the system. The practical result is that inserting a Barlow lens magnifies the image.
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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.[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. Most commonly, Barlow lenses are 2x or 3x but adjustable Barlows are also available. The power of an adjustable Barlow lens is changed by adding an extension tube between the Barlow and the eyepiece to increase the magnification.
The amount of magnification is one more than the distance between the Barlow lens and the eyepiece lens, when the distance is measured in units of the focal length of the Barlow lens. A standard Barlow lens is housed in a tube that is one Barlow focal-length long, so that a focusing lens inserted into the end of the tube will be separated from the Barlow lens at the other end by one Barlow focal-length, and hence produce a 2x magnification over and above what the eyepiece would have produced alone. If the length of a standard 2x Barlow lens' tube is doubled, the lenses are separated by 2 Barlow focal lengths and it becomes a 3x Barlow, if the tube length is tripled, the lenses are separated by 3 Barlow focal lengths and it becomes a 4x Barlow, and so on.
A common misconception is that higher magnification equates to a higher-quality image. However, in practice, the quality of the image generally depends on the quality of the optics (lenses) and viewing conditions, not on magnification.
Teleconverters are variations on Barlow lens that have been adapted for photographic use[2]. It increases the effective focal length of a photographic lens it is attached to, making it a telephoto lens. A true telephoto lens uses a configuration similar to a Barlow lens to obtain a shorter tube length for a given focal length.
In microscopy the Barlow lens is used to increase working distance and decrease magnification. The lenses are "objective lenses" that are mounted in front of the microscope's last objective element. Standard lenses are 2× which decreases the working distance by half and increases the magnification, 0.75× which increases the working distance proportionally to the 0.75 and decreases the magnification similarly. A 0.5× would double the working distance and halve the magnification.