How the eye works

Vision begins with rays of light entering the eye through the cornea (the first transparent tissue) and going through the pupil (the round opening in the middle of the iris which is the colored part of the eye).  The pupil’s size can vary with the amount of light entering the eye.  The rays of light then go through the crystalline lens that, by changing shape, focuses the rays according to the observed object.  Finally, the light hits the retina and its energy excites the retinal cells, creating a nervous influx that travels through the optic nerve towards the brain to create the image that we see.

Visual Acuity (VA)
This is the capacity to distinguish details.  VA is measured using a letter chart.  The results are noted using either the metric system (6/6) or the imperial system (20/20).  Therefore, to have a VA of 6/12 (or 20/40) means that one has to be at 6 metres (or 20 feet) to see what is usually seen at 12 metres (40 feet).  This is what measures your vision capacity.

Myopia (Nearsightedness)
This vision defect occurs when the image of far objects is formed in front of the retina instead of on the retina itself.  Consequently, the far objects are blurred but objects that are closer can remain clear.  Squinting is a common sign of myopia because it helps to clear the vision.

Hyperopia (Farsightedness)
This vision defect occurs when the image of near objects is formed behind the retina.  Therefore, the eye has to make an effort to see both far and near, creating a visual strain or blurriness at both distances.  The common symptoms are difficulty concentrating, difficulty to maintain clear images when looking at close objects for a prolonged period of time and tiredness or headaches after prolonged effort.

Astigmatism
This is a vision defect created by an irregular curvature in the different parts of the crystalline lens and /or cornea which gives blurry vision.  As a result, the  blurriness or distortion at all distances can cause headaches, ocular discomfort and ocular fatigue, variable with the degree of astigmatism.

Presbyopia
This is the loss of the amplitude of accommodation that generally appears towards the age of 40.  In other words, the crystalline lens loses its elasticity and its capacity to focus on near images.  Therefore, reading material has to be held further than the normal distance.  Common symptoms of presbyopia are blurry vision at close distances, headaches, visual fatigue and difficulty to work close-up for long periods.

Strabismus
This is the visual condition in which the two eyes are not aligned properly.  In certain cases, one eye is the fixating eye while the other one turns outside (exotropia) , inside (esotropia) or vertically (hypertropia).  Strabismus usually appears in early childhood but can appear later, depending on the cause.  Treatments include wearing glasses, exercises and even surgery.

Amblyopia
Sometimes called a “lazy eye”, amblyopia is the visual state in which vision is reduced in one or both eyes even if an optical correction is worn.  Amblyopia is often associated with either strabismus or a unilateral vision defect that isn’t corrected when needed.  Glasses and exercises can help.  Early detection (preferably before 6 years old ) is the most important part of the treatment.

Glaucoma
Glaucoma is a disease of the eye that causes a slow and progressive destruction of the optic nerve.  This disease can originate from a surplus of liquid (aqueous humor ) produced in the eye, or a bad evacuation of this liquid, or by difficult blood irrigation of the optic nerve secondary to an inadequate intraocular pressure.  If not treated, this disease can lead to blindness.

Cataracts
With age, with trauma, medication side effects or radiation (UV), the crystalline lens opacifies gradually and causes decreased vision.  Cataracts can create halos, light sensitivity and discomfort in sunlight.  The only treatment is surgery.

Macular degeneration
The macula is the part of the eye that has the most precise vision.  Twenty-five percent of persons over the age of 65 experience a drying of that zone, causing a progressive decrease of vision.  In rare cases, bleeding can occur in the back of the eye. Treatment usually includes frequent eye exams and antioxidant tablets even if the condition is incurable.

Floaters
Floaters are particles made of protein or other natural material that float in the internal liquid of the eye.  If these particles are big enough, they can cause shadows that give the impression of “seeing things” in front of our eyes.  Floaters can also be caused by a deterioration of the ocular fluid (vitreous).  If flashes of light accompany the floaters, this could be a sign of retinal problems.  Either way, floaters and/or flashes should be checked by an optometrist.

Tips

• See your optometrist regularly.  With time, your eyes change and a regular complete eye exam can detect potential problems before they become too serious.  Your optometrist will advise you on the required frequency of your exams.
• Always use an adequate optical correction for hobbies, sports and work.  Discuss these with your optometrist
• Don’t use other people’s glasses.  Each prescription is unique.
• Only use quality lenses.  Distorted lenses can cause visual damage.
• See your family optometrist.