A Barlow lens, also referred to as a diverging lens, is a concave lens that is used in various optical systems to increase magnification.
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.
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 system.
The amplification factor in this case can be calculated using the following formulae:
A - This is the amplification factor being calculated
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 combination.
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 magnification.
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 specimen.
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 lab work.
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 good reputation.
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 problems.
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.
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 eyepiece field.
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 others.
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 enhanced benefits.
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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Hartshorn, C. R. (1953). "The Barlow Lens". In Ingalls, Albert G. Amateur Telescope Making, Book Three. Scientific American. pp. 277–286.