An auxiliary lens (also called a “Barlow” lens or "objective" lens, though this should be distinguished from the built-in objective lens that comes with the microscope) is used to increase or decrease the total magnification of a microscope, or increase or decrease its working distance. A stereomicroscope with a 2x built-in objective lens and 10x eyepieces, for example, would have a total magnification of 20x, but if you needed more than that you could attach a 1.5x auxiliary lens to achieve 30x.

Auxiliary lenses also cause a corresponding increase or decrease in working distance. Mounting a lens that increases total magnification would result in a shorter working distance, while mounting one that decreases magnification would result in a longer working distance. The latter can be helpful for users who need more room for instruments or tools between the specimen and objective lens.

Below are some things to consider when using an auxiliary lens:

1. Like any change in magnification, your field of view would be affected when mounting an auxiliary lens. Decreasing the magnification would let you see more of the sample, and vice versa.
2. If you intend to use a polarizing attachment, wave plate, or other accessory, these are often not compatible with the additional lens. You may have to switch between the two.
3. For microscopes with a camera mounted to a trinocular tube, an auxiliary lens would affect the image seen at both the eyepieces and the camera. This is important if your goal is to change the total magnification, because merely changing the eyepieces would not affect the image at the camera end.

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