Eye and vision conditions
Optically, the light that enters a myopic eye is focused too close to the front of the eye (see figure below). Concave lenses refocus the light on the retina. When a concave lens of the right strength is placed in front of the myopic eye, the light rays are refracted to focus on the retina.
Visual acuity
This is the ability to distinguish details. It is measured with the common eye-chart test. Results are generally given on a scale of 6 (6/6 in the metric system) or 20 (20/20, in the English system). Visual acuity of 6/12 (or 20/40) means the individual can only see at 6 metres (or 20 feet) what can normally be seen at 12 metres (40 feet). This measurement indicates visual capacity.
This is the ability to distinguish details. It is measured with the common eye-chart test. Results are generally given on a scale of 6 (6/6 in the metric system) or 20 (20/20, in the English system). Visual acuity of 6/12 (or 20/40) means the individual can only see at 6 metres (or 20 feet) what can normally be seen at 12 metres (40 feet). This measurement indicates visual capacity.
Emmetropia
Emmetropia is not a vision problem but the "normal" state of the eye. In an emmetropic eye, light focuses on the retina.
Emmetropia is not a vision problem but the "normal" state of the eye. In an emmetropic eye, light focuses on the retina.
Myopia (nearsightedness)
A nearsighted person has trouble seeing far away, but near vision is generally good. Myopia usually begins between the ages of 6 and 21 and stabilizes around the age of 30. Since the myopic eye is optically too powerful, its strength must be reduced using negative lenses.
Clinically, nearsightedness is a refraction problem that requires concave lenses (to shrink the image) in order to achieve normal vision.
A nearsighted person has trouble seeing far away, but near vision is generally good. Myopia usually begins between the ages of 6 and 21 and stabilizes around the age of 30. Since the myopic eye is optically too powerful, its strength must be reduced using negative lenses.
Clinically, nearsightedness is a refraction problem that requires concave lenses (to shrink the image) in order to achieve normal vision.
Optically, the light that enters a myopic eye is focused too close to the front of the eye (see figure below). Concave lenses refocus the light on the retina. When a concave lens of the right strength is placed in front of the myopic eye, the light rays are refracted to focus on the retina.
Hypermetropia (farsightedness)
For a farsighted person, the theoretical point of convergence of the light is behind the retina, because the accommodation is too lax. The farsighted person can correct this situation by accommodating, that is, increasing the strength of the eye and forcing the light to focus on the retina. This constant effort to focus to achieve clear vision can be uncomfortable and eventually lead to the following problems:
Farsightedness can be corrected with convergent (or convex) lenses. These lenses have a magnifying effect, so eyes seen through them appear bigger than they really are.
Optically, the light that enters a hypermetropic eye is focused too far away (see figure below). Convex lenses refocus the light on the retina.
When a convex lens of the right strength is placed in front of the hypermetropic eye, the light rays are refracted to focus on the retina. See figure.
Hypermetropia should not be confused with presbyopia, which is characteristic of adult eyes beginning at age 40 or 45.
For a farsighted person, the theoretical point of convergence of the light is behind the retina, because the accommodation is too lax. The farsighted person can correct this situation by accommodating, that is, increasing the strength of the eye and forcing the light to focus on the retina. This constant effort to focus to achieve clear vision can be uncomfortable and eventually lead to the following problems:
- Headaches (especially at the and of the day)
- Tearing and reddening of the eyes
- Photophobia (oversensitivity to light)
- Trouble with close vision
- Blinking
Farsightedness can be corrected with convergent (or convex) lenses. These lenses have a magnifying effect, so eyes seen through them appear bigger than they really are.Optically, the light that enters a hypermetropic eye is focused too far away (see figure below). Convex lenses refocus the light on the retina.
When a convex lens of the right strength is placed in front of the hypermetropic eye, the light rays are refracted to focus on the retina. See figure.
Hypermetropia should not be confused with presbyopia, which is characteristic of adult eyes beginning at age 40 or 45.
Astigmatism
Astigmatism is when light from a single point converges on two different places on the retina. This anomaly is usually caused by a lack of uniformity in the curve of the cornea (anterior surface of the eye): the astigmatic eye is oval, like a football, rather than round.
Astigmatism first appears in childhood and is not subject rapid changes. The patient may complain of eye pains, headaches, eye strain and blurry vision to varying degrees depending on the severity of the astigmatism. A patient wearing corrective lenses for astigmatism for the first time may have the impression that objects are deformed or bent, because the eye is used to seeing things in a particular way. The brain takes time to readapt, but after a few weeks everything gets back to normal.
Astigmatism is when light from a single point converges on two different places on the retina. This anomaly is usually caused by a lack of uniformity in the curve of the cornea (anterior surface of the eye): the astigmatic eye is oval, like a football, rather than round.
Astigmatism first appears in childhood and is not subject rapid changes. The patient may complain of eye pains, headaches, eye strain and blurry vision to varying degrees depending on the severity of the astigmatism. A patient wearing corrective lenses for astigmatism for the first time may have the impression that objects are deformed or bent, because the eye is used to seeing things in a particular way. The brain takes time to readapt, but after a few weeks everything gets back to normal.
Presbyopia
Presbyopia is a normal physiological phenomenon that appears between the ages of 40 and 50 years old. As the crystalline lens grows older, it loses some of its elasticity, which causes its power of accommodation to decline. This means the person has greater difficulty seeing objects close by. Reading is difficult and uncomfortable for people with presbyopia, and they often hold their reading matter at arm's length or increase the ambient light to try to compensate.
Presbyopia is corrected with convex lenses. For people who wear glasses already, a convex lens must be added to the initial prescription to create a bifocal lens. If the only problem is presbyopia, a number of different solutions are possible:
Presbyopia is a normal physiological phenomenon that appears between the ages of 40 and 50 years old. As the crystalline lens grows older, it loses some of its elasticity, which causes its power of accommodation to decline. This means the person has greater difficulty seeing objects close by. Reading is difficult and uncomfortable for people with presbyopia, and they often hold their reading matter at arm's length or increase the ambient light to try to compensate.
Presbyopia is corrected with convex lenses. For people who wear glasses already, a convex lens must be added to the initial prescription to create a bifocal lens. If the only problem is presbyopia, a number of different solutions are possible:
- Reading glasses, with the entire lens devoted to correcting near vision; these glasses will interfere with the person's distance vision.
- Half glasses that correct presbyopia without disturbing distance vision, because the wearer looks over them to see objects at a distance.
- Progressive lenses provide improved vision at all distances thanks to a gradual change in lens strength from one part of the lens to another.
Strabismus
This is when the eyes are not fully aligned. Usually when we look at an object, both eyes are directed toward the fixation point.
Sometimes, one eye fixates on this point while the other turns either outward (exotropia), inward (esotropia) or up or down (hypertropia). Strabismus usually appears in early childhood but may occur later, depending on the cause. Treatment may include glasses, exercises or even surgery.
People with strabismus have problems with binocular vision (three-dimensional vision) and may sometimes suffer from diplopia (double vision). If there is constant or occasional deviation from the axis, a specialist should be consulted immediately for an in-depth examination of the visual system.
This is when the eyes are not fully aligned. Usually when we look at an object, both eyes are directed toward the fixation point.
Sometimes, one eye fixates on this point while the other turns either outward (exotropia), inward (esotropia) or up or down (hypertropia). Strabismus usually appears in early childhood but may occur later, depending on the cause. Treatment may include glasses, exercises or even surgery.
People with strabismus have problems with binocular vision (three-dimensional vision) and may sometimes suffer from diplopia (double vision). If there is constant or occasional deviation from the axis, a specialist should be consulted immediately for an in-depth examination of the visual system.
Amblyopia
Amblyopia, or lazy eye, is when the vision is reduced in one or both eyes even when corrective lenses are worn. Amblyopia often accompanies strabismus or a unilateral vision problem that was not corrected in time. Glasses and exercises may improve amblyopia.
Amblyopia, or lazy eye, is when the vision is reduced in one or both eyes even when corrective lenses are worn. Amblyopia often accompanies strabismus or a unilateral vision problem that was not corrected in time. Glasses and exercises may improve amblyopia.