Visual Optics

How the Eye Works

The eye works like a very elegant biological camera. Light from the outside world is focused by the combined effect of the cornea and the lens, so that a sharp image is created on the retina..

In an eye with no need for distance prescription, light is brought into sharp focus exactly on the retina. Scroll over any term in the left pane of animation below, and the corresponding part of the eye highlights yellow in the right pane. The reverse works too: Point to a part of the eye in the right pane and the corresponding term is highlighted.

(Note:Flash animations will not display on iOS devices (iPhone, iPad), as Apple currently does not support Flash)

 

 

Point of view with normal vision, no optical imperfection
Normal Vision: Clear Far and Near
 

The optically optimal eye of a young person, with zero prescription need, can see clearly at all ranges, near and far. 

Nearsightedness (myopia)farsightedness (hyperopia) and astigmatism are all imperfections of distance vision.  Presbyopia describes the loss of close focusing that occurs as we enter our mid-40s.

Nearsightedness (myopia)

 

 

Nearsightedness or myopia - point of view
Nearsighted View: Clear Close, Blurry Far

Nearsightedness (myopia) is the most common optical condition in adults worldwide, affecting about 35% of the population. In the nearsighted eye, light from distant objects focuses in front of the retina, not on it; so images are blurry. 

Correcting Nearsightedness by laser treatment involves precisely sculpting the desired correction into the cornea. Glasses and contacts work by adding a concave lens to the eye, whereas laser treatment works by evaporating or subtracting a convex lens shape from the cornea.

Farsightedness (hyperopia)

 

 

Farsighted View: Better Far Than Close
Farsighted View: Better Far Than Close

Farsightedness (hyperopia) is the optical opposite of nearsightedness. In a farsighted eye, light focuses behind the retina, so a blurry image is perceived. Farsightedness affects about 10% of the adult population.

Farsightedness correction by laser involves sculpting to steepen central corneal curvature. Glasses and contacts correct this by adding a convex lens to the optical system, whereas laser treatment works by evaporating or subtracting a concave lens-shape from the cornea.

Astigmatism

Astigmatism is one of those strange terms that most people don't understand, so it sounds intimidating when you are told that you have it. It doesn't help that prescription contact lenses cost significantly more if astigmatism is present. Let's clear up some of this confusion.

 

Point of view with astigmatism
Astigmatism: Blur, Ghosting, Doubling
or "Drop-Shadow"

Astigmatism describes optical blur that arises when the curvature of an optical surface is "out of round". This irregular curvature can occur on the cornea or on the lens.

Astigmatism Symptoms

Optical blurring, ghosting, slight doubling or drop-shadow effects result from having astigmatism. Most people only know they have astigmatism because their eye doctor has measured them for glasses or contacts and has told them they have this condition.

 

 

How can Astigmatism be Treated?

Regular astigmatism may be corrected through:

  • Prescription eyeglasses
  • Prescription contact lenses
  • LASIK
  • Lens replacement surgery using an astigmatism-correcting lens implant ("IOL")
  • Placement of arcuate incisions in the peripheral cornea, typically done at the time of cataract and IOL surgery ( also called limbal relaxing incisions)

Lenses called 'toric lenses' are prescribed to correct astigmatism. LASIK, when properly rendered with modern lasers, can beautifully correct regular astigmatism.

Cataract and Clear Lens Replacement patients who have astigmatism can be treated with a toric Intra-Ocular lens (IOL) implant during cataract surgery to correct pre-existing astigmatism.

Irregular astigmatism, affecting 1% of people, is a more serious condition. This is more challenging to treat and is discussed in detail on our High Astigmatism page.

Dr. Wallace has performed literally thousands of surgical treatments for astigmatism and will review your case carefully to recommend and treat you with the very best solution for your visual goals.

Contact our office today at (310) 828-2020, or Contact Us online to request a free astigmatism-correction consultation.

Presbyopia

If you look through a single lens reflex camera (without using the auto-focus function), the barrel of the lens rotates to adjust focus. If you focus at a distance, things close up are blurry, and vice versa. The human eye works in similar fashion. The optically perfect eye (with zero prescription need for distance, called "emmetropic") sees distant objects easily. To look at something up close, focus is adjusted by a process called accommodation. A circular muscle behind the iris contracts and moves forward slightly. This muscle is connected to the lens by tiny fibers called zonules. The change in tone and position of the ciliary muscle changes the tension of the zonules on the lens. This "tug" causes the lens to change curvature, increasing focusing power.

In our youth, the lens is flexible and accommodates easily. As we mature, layers of protein are laid down just inside the outer lining membrane of the lens in a fashion similar to the formation of rings on a tree trunk, causing the lens to become thicker, denser, and less flexible. At a certain age, close focusing becomes difficult, and we need magnifying glasses to see near objects without eyestrain. This condition, called presbyopia, usually begins in our early- to mid-40s and gets progressively worse for about 10 years, then stabilizes.

Presbyopia is a natural process related to the normal aging of the human lens.  While there is no perfect cure for this, there are several treatment options available.  See our section on Reading Vision to learn more.