A Barlow lens is a diverging lens that alters the focal length of a microscope and, therefore, the field of view. They also alter the working distance between the objective lens and the specimen, which is a critical variable for many applications such as PCB soldering and inspection.
A Barlow attaches to the bottom of the objective lens on a stereo (low power) microscope when there is a need for either more or less of one of the following variables:
Field of View
The most common type of Barlow lens is the Reducing Barlow. A Reducing Barlow reduces the magnification power of the microscope, but has the advantage of increasing the field of view and the working distance between the objective and the specimen. Reducing Barlows are typically 0.3x, 0.5x and 0.75x although other powers are available.
As an example, a 0.5x Barlow lens will halve the magnification power of the microscope, but it will double the size of the field of view. It will also increase the working distance between the objective and the specimen.
By the same token, a 2.0x Barlow will double the magnification and halve the field of view of a microscope. In other words, each different power Barlow has a proportionate effect on magnification and a direct inverse proportional effect on the field of view and working distance, as shown below.