A Barlow lens, also referred to as a diverging
lens, is a concave lens that is used in various optical systems to increase
Named after the English mathematician and
physicist Peter Barlow, it is typically used in such optical systems as
microscopes, telescopes and cameras. In such systems, the lens is placed
between the objective lens, or a primary mirror and the eyepiece. In this
position, the lens increases the effective focal length of the objective thus
increasing the overall magnification.
Understanding the Barlow Lens
Essentially, the Barlow lens is a negative lens.
This simply means that it is the type of lens that causes a beam of parallel
rays of diverge after reflection. As such, this lens is placed a short
distance inside the focus of the objective.
Being a diverging lens, the lens
significantly increases the focal length from moving the actual focal point
only a small distance. The typical Barlow lens is also an achromatic that is
composed of Plano-convex flint cemented to a bi-concave crown while at the same
time having a negative focal length of the order of 100mm. Placed 50mm in the
focus of the primary objective, this lens doubles the focal length of the
The amplification factor in this case can be
calculated using the following formulae:
A - This is the amplification factor being
F - This is the Barlow focal length (here, the
negative sign is ignored)
d - d is the distance of the Barlow inside the
focal point of the mirror/objective
The amplification factor refers to the function
of position of the lens in relation to the eyepiece and the objective lens. For
given eyepiece and objective, the separation between the Barlow lens and
eyepiece and the Barlow and the objective are related given that the focal
plane of the eyepiece is the same as the focal plane of the objective-Barlow
With increasing separation between the eyepiece and the
Barlow, separation between the Barlow and objective decreases. Therefore, it
becomes possible to increase the amplification factor of the Barlow simply by
increasing the separation between the lens and the eyepiece using the
extension tube while bringing it close to the objective.
In a microscope, these lenses can either be
used to amplify or reduce magnification. To use the lens, in a stereo
microscope, it has to be screwed on to the objective lens housing in order to
change the magnification. For a stereo microscope, it also serves to change the
working distance that is needed to focus.
For such microscopes as the AmScope
SM series, a 0.5x Barlow, 2.0x Barlow or both can be used. Here, if the 0.5x
Barlow is used, then the magnification is reduced by half. In the
event that a 2.0x Barlow lens is used, then the magnification is doubled. Therefore,
choosing a Barlow lens will largely depend on what the user needs.
While this lens is very useful for either
increasing or decreasing the magnification, it can be particularly useful when
a camera has been mounted onto a trinocular port of a microscope. Here, it is
important to note that when a camera has been mounted on to a trinocular
microscope, then switching the eyepieces does not result in changes in the
However, by adding a Barlow lens, it becomes possible to change
the magnification in both the eyepieces as well as the camera. It
can be beneficial to include it when using a camera on a trinocular
microscope particularly if one wishes to change the magnification when viewing
The other useful quality of the Barlow lens is
changing the working distance. In microscopy, the working distance refers to
the right distance between the top surface of the sample/specimen and the bottom
of a microscope lens (objective) for the sample to be in focus.
It becomes easier to change this distance to a more desirable
distance while focusing when using this lens. Increasing the working distance, particularly with
stereo microscopes, is important for such activities as soldering and dental
Barlow Lens Buyer's Guide
Some microscopes come with a Barlow lens already
in place. However, others do not and even when a microscope comes with the
lens, the user may wish to change it.
The following are important factors to
consider when buying a microscope Barlow lens:
Quality - For most first
time users, Barlow lenses have gained a bad reputation for poor quality.
However, this is largely due to the fact that some users get poor quality,
plastic, toy-like lenses that will definitely provide poor image quality. When
looking for such a lens, it is important to buy precision
multi-lens and all-glass optical systems. It is important to purchase from a brand with a
Apochromatic vs. Chromatic - Like microscope
objectives, Barlow lenses are also divided into achromatic systems and
apochromatic systems; where the achromatic lenses have two lenses while the
apochromatic lenses have three. Although both will provide good services,
apochromatic lenses are superior. Here, it is important to know the two
different types before making the decision.
Mounting Size - Different Barlow lenses
are of different sizes. It is therefore important to make sure that one
understands the appropriate size required to mount to the microscope. This
ensures that the right fit is obtained. It is always important to avoid forcing
any attachment on to the microscope in order to avoid damage.
In addition, remember that such accessories as the Barlow lens may need to
be replaced with time. Therefore, getting the right fit ensures that it mounts
without any problems and when it comes time to replace it, that there are no
Amplify vs. Reduction - These lenses are not
only used for increasing magnification, but also for reduction purposes. It is
imperative to understand what different Barlow lenses do before making
the purchase. For instance, while a 0.5x lens will result in reduced magnification,
a 2x will increase magnification. For this reason, knowing the intended purpose
of the lens will help get the appropriate lens for the intended purpose.
In addition, the Barlow lens is also
used to protect the objectives of some microscopes.
One of the biggest advantages
of these lenses is that they allow for the expansion of the range of
magnifications without the need to invest in different eyepieces. With a
trinocular stereo microscope (when using a camera), this is particularly
beneficial given that changing the eyepieces alone does not result in any significant
With regards to ergonomics,
the use of these lenses is beneficial given that they allow for more comfort
when it comes to observing/viewing the sample at higher power with long focal
Although there are
different qualities, good quality lenses can help improve the
image quality particularly when it comes to sharpness of the image. This requires the combination of a good quality Barlow lens and the appropriate
They can improve the edge sharpness of good
quality wide angle eyepieces while providing inexpensive eyepieces a
slower-converging and easier to handle light cone. This has been shown to
result in lower astigmatism and better color correction at the edge of the
Stereo Microscope Picks
Although a Barlow lens is not necessarily
essential for a microscope to work, they are important in that they can serve a
number of functions including increasing magnification, reducing magnification,
increasing the working distance as well as protecting the objectives among
As is the case with all the other lenses used in
microscopes, it is important to ensure that one gets the right specific lens for
Barlow lenses are also used in other optical systems like
telescopes and cameras in photography. For this reason, it is important to get
the right lens for your microscopy purposes. Once the
right lens has been obtained, the viewing experience will be enhanced.
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