Your ability to see is dependent on the function of several different anatomical components of the eye. While all of the eye's parts play a role in your vision, the most important are the iris, pupil, cornea, lens, retina and optic nerve.
The iris is the colored part of your eye that surrounds the pupil.
The pupil is the center dark portion of your eye. It plays an integral component in vision as it is responsible for regulating how much of the light entering your eye makes its way to the retina.
The cornea covers the front of the eye and is responsible for refracting (bending) the majority of entering light rays. Light rays enter the eye after being reflected from the object of focus. Although the cornea's shape does not change drastically over time, small alterations will occur as you grow older. The cornea is responsible for approximately 2/3 of the focusing power of the eye.
Like the cornea, the clear lens of the eye plays an important role in making sure that light rays are properly focused on the retina. The lens is able to change its shape quickly to help the eye focus on objects at various distances. A common disease of the lens is called cataract, which occurs when the lens becomes clouded. The lens is responsible for approximately 1/3 of the focusing power of the eye.
The retina is a collection of nerve tissue located at the back of the eye that contains rods and cones. These specialized cells convert the focused light energy coming from the pupil, cornea and lens into nerve signals. These signals are carried from the retina to the optic nerve, and then to the brain.
The optic nerve carries nerve signals from the retina to the brain, where they are translated.
Refractivity is the term to describe how light rays are focused in the eye after being reflected from objects into the field of vision. The eye is composed of several complex components to properly interpret, focus and process light into nerve impulses. These nerve impulses are then sent to the brain, which is how we see. There are several aspects of the eye that determine the quality of vision, including the shape of the cornea, the power of the eye's lens and the length of the eye. These three visual components must work together to properly determine how light entering the eye is focused on the retina. When they do not function properly, vision quality is affected.
Several conditions can affect the eyes' ability to focus properly. Hyperopia occurs when the cornea is too flat or if the eye is too short. Also known as farsightedness, hyperopia causes near objects to appear blurry. If the eye is too long in relation to the curvature of the cornea, or if the cornea is drastically curved in relation to the length of the eye, myopia will result. Myopia, more commonly referred to as nearsightedness, is a condition where individuals are able to see close objects clearly but objects in the distance appear blurry. Astigmatism can occur with both myopia and hyperopia. Astigmatism will distort images due to an irregularity on either the cornea or the lens.
Most people are familiar with the Snellen chart that is used to measure visual acuity. It is the large chart, typically with a large 'E' at the top, in the examination room. This chart contains several rows of letters, diminishing in size from top to bottom. Your ability to identify the letters allows the doctor to determine your visual acuity. There are numerous levels of visual acuity that people are familiar with. 20/20 vision means that from 20 feet away you are able to accurately read the designated line. Individuals with 20/40 vision can accurately read a designated line from 40 feet that people with 20/20 vision can read from 20 feet.
20/20 vision occurs naturally when all of the eye's components are properly aligned and working together. Light rays reflecting off of objects in your field of vision enter each eye and are bent (refracted) by the cornea and lens. In a properly functioning eye, the light that passes through the cornea and lens creates a distinct image on the retina. (The retina is a layer of nerve tissue that contains rods and cones that are responsive to light.) Some people's eyes improperly transmit the light from the cornea and lens to the retina. This is known as a refractive error, and causes images to be blurry on the retina. The extent to which images are blurred determines the patient's level of nearsightedness, farsightedness and/or astigmatism. There are numerous methods used to correct refractive errors, including laser eye surgeries such as LASIK and traditional methods such as glasses and contact lenses.
A diopter is a unit of measurement for an individual's level of refractive error. Diopters are expressed in positive numbers for farsighted patients and in negative numbers for nearsighted patients. For example, if your myopic measurement is -1.00, then you are able to see objects clearly at one meter. If you have a -2.00 myopic measurement, then you are able to see objects clearly at 0.5 meters. The greater the myopia you have, the stronger the corrective lens you need to compensate for your eye's refractive error.
Approximately 25 percent of all Canadians suffer from some degree of myopia, or nearsightedness. Patients with -1 to -2 diopters of nearsightedness are considered mildly myopic while those with prescriptions ranging from -4 to -6 diopters are considered to be moderately nearsighted. Patients with prescriptions greater than -8 diopters are described as high levels or severely myopic. There are several misconceptions about myopia, including the idea that it can be caused by reading for long periods of time, reading in lowlight conditions, wearing glasses that are too strong or weak or other similar activities.
Studies have shown that individuals with myopia have a genetic disposition to the condition. It is usually discovered in their early teens and typically worsens as the length of the eye grows during puberty. Most individuals experience a stabilization of the condition when they become adults.
In a myopic eye, the cornea is disproportionately curved in relation to the length of the eye. This structural difference causes light rays to be focused in front of the retina instead of on this nerve tissue, which causes distant objects to appear blurry or fuzzy. The greater the myopia, the more the light rays converge in the wrong spot, causing objects in the distance to appear even more blurry or fuzzy.
A myopic individual can see an object clearly when it is within his or her range of vision not requiring correction. As mentioned before, if you have a myopic measurement of -2.00, you are able to see everything clearly within 0.5 meters. The eye's ability to properly focus within this range allows many mildly myopic individuals to remove their glasses while completing certain tasks such as reading or working on crossword puzzles.
As with every vision correction option, there are advantages and disadvantages to correcting myopia with glasses. The main advantage of wearing glasses is that it allows a myopic individual to see distant objects more clearly. But there are several disadvantages to wearing glasses, including aesthetic reasons, minification and peripheral vision problems. Minification, or visual size reduction, stems from the concave shape of the lenses used to correct myopia. Studies have shown that there is an image minification of approximately 2 percent for every diopter of spectacle correction. For example, a -5 diopter spectacle correction will result in roughly 10 percent reduction in image size. Glasses with thick frames and lens edges can also distort and/or reduce peripheral vision.
Contact lens wearers typically do not have the optical problems, including minification and peripheral vision problems, that spectacle wearers typically do. They can, however, experience other disadvantages such as corneal warping, corneal infections and eyelid allergies. Some contact lens wearers become intolerant of contact lenses and are unable to wear them anymore. This typically occurs in patients who are 40 years of age or older and among patients who have dry or sensitive eyes.
There are many different options for the surgical correction of myopia, including LASIK, PRK and the Implantable Contact Lens. To learn more about the treatments we offer, visit our Vision Correction Surgery page.
Hyperopia, also known as farsightedness, occurs in approximately 25 percent of the population. Individuals with this condition can see distant objects clearly but have difficulty seeing objects up close. A hyperopic individual usually has a diopter measurement between +1.00 and +4.00. In extremely rare cases, patients can have diopter measurements as high as +8.00 diopters. These patients typically suffer from optical aberrations and decreased peripheral vision.
The problem with a hyperopic eye is that the cornea is not angled enough, so light rays entering the eye are not yet focused by the time they hit the retina. This causes close objects to be blurry. Most hyperopic individuals need to wear some form of correction to have clear vision.
Spectacles used to correct hyperopia have convex lenses. This lens style acts as a magnifier to make near objects appear larger. Most prescriptions make objects appear larger at a rate of 2 percent per diopter.
Many hyperopic people have had success correcting their vision using contact lenses. There are several disadvantages contact lens wearers face, including corneal infections, eyelid allergies and corneal warping. Some patients who are older than 40 and have dry or sensitive eyes develop an intolerance for contact lenses.
There are several surgical options to help treat hyperopia, including LASIK, PRK and Refractive Lens Exchange (RLE). Please visit our page devoted to Vision Corrective Surgery to learn more about other treatment options.
Astigmatism is a type of refractive error caused by an irregularly shaped cornea or lens. Patients who suffer from astigmatism typically have an oblong shaped cornea rather than the normal spherical shape. This causes the entering light rays to focus on two spots in the back of the eye instead of a single spot. Astigmatism affects all levels of vision and is usually accompanied by hyperopia or myopia.
Individuals who treat their astigmatism with spectacles require a special type of lens, called a cylinder, to help light focus more precisely on the retina.
Advancements in technology have allowed more astigmatism sufferers to correct their vision by wearing contact lenses. In the past only rigid contact lenses were suitable for astigmatic patients, but now many people are able to use a type of soft contact lenses called toric contact lenses. Toric contact lenses are created with a special prescription that may also contain a prescription for myopia or hyperopia if required.
Our Toronto LASIK surgeons can help people with astigmatism. The treatment technique depends on the type and severity of the condition. To learn more about LASIK and other surgical correction options, visit our Vision Correction Surgery page.
Presbyopia is an age-related eye condition that affects nearly everyone older than 40, even those who previously had never experienced a vision problem. This condition is generally caused by the eye's natural lens slowly losing flexibility. The loss of elasticity makes it difficult for the eye to focus on objects up close. This is why many presbyopia sufferers hold reading materials at arm's length or complain of constant eye strain.
Bifocal eyeglasses are the most common form of treatment for presbyopia. These glasses are able to treat two different types of vision problems. The top part of the lens contains a prescription for hyperopia or myopia, while the prescription of the lower portion is made for viewing objects that are very close. Reading glasses are a popular choice for individuals who do not require any correction for distance vision, but need a prescription to treat presbyopia. Many contact lens wearers who have presbyopia choose this option to avoid continually wearing glasses.
Multifocal contact lenses have been used successfully by many presbyopic individuals. Others choose the monovision approach, where the prescription of one contact lens is set for distant vision correction and the other is set for near vision correction. While this treatment option works well for some people, others have difficulty making the eyes work together. They typically complain about eye strain, dizziness and headaches.
The most important thing to remember about presbyopia is that it is a progressive condition, meaning the severity of presbyopia increases with time and age. Several surgical treatments are available to treat presbyopia, including the Refractive Lens Exchange (RLE )with a bi-focal or accommodative intraocular lenses. Laser Vision Correction with Mono-Vision and NearVision CK ( conductive keratoplasty) are other treatment options to decrease the need for reading glasses for those with presbyopia.
To learn more about your vision needs, contact the Bochner Eye Institute at one of our three locations. Schedule a personal consultation with one of our experienced eye surgeons – Dr. Harold Stein, MD, Dr. Raymond Stein, MD, and Dr. Albert Cheskes, MD – to find out which procedure can help you achieve clear, crisp vision.