The Barlow lens, named for its creator, the English engineer Peter Barlow, is a diverging lens

Lens (optics)

A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 which, used in series with other optics

Optics

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 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.

Telescope use

In its astronomical

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 use, a Barlow lens may be placed immediately before an eyepiece

Eyepiece

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 to effectively decrease the eyepiece's focal length

Focal length

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 by the amount of the Barlow's divergence. Since the magnification

Magnification

Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 provided by a telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 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.

Photography use

Teleconverter

Teleconverter

A teleconverter is a secondary lens which is mounted between the camera and a photographic lens. Its job is to enlarge the central part of an image obtained by the objective lens...

s are variations on Barlow lens that have been adapted for photographic use. It increases the effective focal length of a photographic lens it is attached to, making it a telephoto lens

Telephoto lens

In photography and cinematography, a telephoto lens is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus...

. A true telephoto lens uses a configuration similar to a Barlow lens to obtain a shorter tube length for a given focal length.

Microscope use

In microscopy

Microscopy

Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

 the Barlow lens is used to increase working distance and decrease magnification. The lenses are "objective

Objective (optics)

In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

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.