What Is an Ocular Lens on a Microscope?

The ocular lens, also known as the eyepiece, is the part of a compound microscope into which a user looks to see a magnified image. It is a see-through double convex lens curved to cause images to appear larger. The ocular lens is an essential element to every compound microscope.

Scientist looking through microscope
A scientist looking through a microscope in a lab.
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Description and Location

"Ocular" means of or connected with eyes or vision, and a lens is a piece of glass with curved sides that concentrates light rays and magnifies images. Thus the ocular lens quite simply is the one you look through. It causes the contents of the slide to appear magnified. The curvature of the lens determines the magnification level. On the microscope the ocular lens is found at the top of the tube which connects it to the objectives lens or lenses.

Magnifying Power

Most ocular lenses have either a 10X (magnified ten times) or 15X magnification level. When a person looks through the ocular lens, light reflects from the objective lens, and the slide is magnified by the multiplied magnification levels of both the ocular and the objective lenses. So if a person is using a microscope with an ocular lens of 10X and they set the objective lens to 4X, then the slide will appear 40X when viewed through the ocular lens. Most microscopes have several settings for objective lenses.


A bright light source is essential to the proper use of a compound microscope. It can be a mirror that reflects light onto the object or a light that is attached to the device. When light passes through the object and hits the lens, the object appears magnified. Because of the physics of light waves, the total magnifying power of a compound microscope is limited to about 2,000X. Beyond that point, the image becomes too blurry to have any practical value. To see an image at greater magnification, you need an electron microscope.


The compound microscope was invented around the year 1590 by Zaccharias Janssen, a Dutch spectacle maker. With the help of his father Hans, he discovered that by using multiple lenses and a tube it is possible to greatly magnify the size of an object. This caught the attention of Galileo, who then made vast improvements to both the microscope and the telescope. Though microscopes today have undergone enormous technological advancements, they still use the same basic parts.

Variety of Designs

A basic student microscope typically has a single ocular lens, used by putting one eye up to the eyepiece and closing the other. Binocular microscopes have pair of ocular lenses, providing added comfort for the user. More advanced microscopes used in the classroom may have two or more separate sets of eyepieces, allowing more than one person to observe a specimen at the same time. For example, a student may be looking at blood cells, and a teacher can check the student's observations.