What You Need to Know About the Eye Lens

The lens is a transparent structure in the eye that is suspended immediately behind the iris and that brings rays of light to a focus on the retina. The crystalline lens is the name given to the natural lens that humans are born with. Small muscles attached to the lens can make the lens change shape, which allows the eyes to focus on near or far objects.

anatomy of the eye

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Parts of the Eye

To best understand the function of the crystalline lens, it helps to know the anatomy of the eye. These are the important structures in the eye, from front to back:

  • Cornea: The clear, dome-like surface of the eye that bends light to focus it through the pupil and onto the retina
  • Sclera: The outer white part of the eye that gives the eye its shape and protects its delicate inner structures
  • Pupil: The opening in the center of the iris that regulates the amount of light that reaches the retina
  • Iris: The colorful membrane behind the cornea that adjusts to help the pupil regulate the flow of light
  • Lens: The transparent structure behind the iris that changes its shape to focus light on the retina, allowing you to see details from varying distances
  • Ciliary body: The muscular structure behind the iris that controls the shape of the lens when the eye focuses
  • Retina: The tissue lining the back wall of the eye that translates visual information into an image that is sent to the brain

Lens Function

The crystalline lens provides approximately one-third of the focusing power of the eye. The lens is flexible and its curvature can change by influence of the ciliary body. The lens changes curvature so that the eye can focus on images at different distances. This change in focusing is called accommodation.

When our eye looks at something at a very close distance to us, our ciliary body contracts and this loosens the lens zonules which hold the lens in place as the lens thickens. When the eye looks at images far away, the ciliary body relaxes, the lens zonules tighten back up, and the lens decreases in thickness. This causes images far away to become in focus.

The Lens and Refraction

Refraction, or the bending of light inside the eye, occurs when light travels through lens. The lens focuses images on the retina. If the lens causes the focus to occur behind the retina, farsightedness occurs. If the lens causes the focus to occur in front of the retina, nearsightedness occurs. Wearing glasses or contact lenses can correct these vision problems. 

Lens Aging and Problems

Many adults will start to notice changes in their vision in their early to mid-40s, and these changes can vary drastically from person to person. As the lenses in our eyes age over time, they can gradually lose their function. Two common conditions that occur as a result of this are presbyopia and cataracts.


Aging causes the lens to become less flexible and elastic. As a result, the eye loses some of its ability to focus on near objects. This condition is known as presbyopia. At around the age of 40, most people require reading glasses. Presbyopia is often referred to as "short arm syndrome" because people tend to hold reading material out away from the body to read it more easily.

People who have presbyopia may complain of needing more light to read. Presbyopic eyes also feel fatigued and seem to tire more easily. Some presbyopic patients may also have fluctuating vision, as their eyes are attempting to overcompensate for the vision deficit.


A cataract is a clouding of the lens. Cataracts often develop as we age. Fortunately, cataracts grow slowly and may not affect vision for several years. By age 65, over 90% of people have a cataract. 

Cataract treatment involves replacing the cloudy lens with a new, clear lens implant in its place. The procedure is usually performed under local anesthesia on an outpatient basis, taking less than an hour in most cases.

Natural vs. Artificial Lenses

To treat cataracts, there are several types of artificial lenses—or intraocular lens implants (IOL)—that can be surgically implanted in place of a cloudy lens. The type of artificial lens that will be ordered for you depends on your visual needs. There are two types of IOLs that are most widely used to correct cataracts:

  • Monofocal lenses, which are designed to correct vision at one distance, are the most common artificial lenses for correcting cataracts. They are typically chosen to correct either nearsightedness or farsightedness.
  • Multifocal lenses, which correct multiple distances, allow you to see clearly at all distances, and most closely simulate normal vision. These work together with the ciliary body to let you focus in and out naturally.

Cataract surgery is safe and effective, and once in place, IOLs can enhance your vision for a lifetime without clouding or moving. Side effects from this procedure are very rare, though some people may develop swelling, eye infection, bleeding, or retinal detachment.

Frequently Asked Questions

What is the crystalline lens made of?

The lens is made of specialized cells that are stacked like layers of an onion. The cells are densely packed with crystalline proteins—about 60% of the lens is protein, a higher concentration than any other tissue in the body—but the cells have no blood supply, no organelles, and no metabolic activity.

What kind of image does the lens form on the retina?

When external light passes through the cornea, the light is bent onto the crystalline lens. The crystalline lens bends the light a second time and an inverted image is formed on the retina. When that image reaches the brain, the image is turned upright.

A Word From Verywell

The crystalline lens is a small but powerful structure that is vital to your ability to see clearly. Like all other parts of the body, these lenses age over time and can gradually lose function. If you are having trouble reading or seeing in low light, connect with an eye doctor to learn more about the corrective options available to you. With the right course of action, you may see significant improvements and enjoy the fine details of life again.

7 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  3. Kellogg Eye Center. Michigan Medicine. Anatomy of the eye.

  4. Kellogg Eye Center. Michigan Medicine. Cataract.

  5. Breyer D, Kaymak H, Ax T, et al. Multifocal intraocular lenses and extended depth of focus intraocular lensesAsia Pac J Ophthalmol (Phila). 2017 Jul;6(4):339-349. doi:10.22608/APO.2017186

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By Troy Bedinghaus, OD
Troy L. Bedinghaus, OD, board-certified optometric physician, owns Lakewood Family Eye Care in Florida. He is an active member of the American Optometric Association.