What does 20/20 vision mean?
Visual acuity is usually measured with some form of Snellen eye chart. The eye chart displays lines of letters, numbers or pictures of progressively smaller sizes. “Normal” vision is 20/20 which means the person being tested sees the same line on the chart at 20 feet away as a person with “normal” vision would see at 20 feet. 20/40 vision means that a person sees at 20 feet away what a person with “normal” vision would see from 40 feet away. The higher the second number, the worse the vision is. For example, 20/100 is worse than 20/50. Visual acuity can also be measured in metric units. In this case “normal” vision is 6/6 which refers to vision at 6 metres.
What is myopia?
Myopia is commonly referred to as “nearsightedness”. Myopia occurs when light entering the eye focuses in front of the retina (photosensitive nerve layer at the back of the eye). This happens when the cornea (clear dome that covers the front of the eye) is too curved or the length of the eye is too long. Near objects are seen clearly but distance objects appear blurred. Myopia can be corrected with glasses, contact lenses or laser surgery.
What is hyperopia?
Hyperopia is commonly referred to as “farsightedness”. Hyperopia occurs when light entering the eye focuses behind the retina (photosensitive nerve layer at the back of the eye). This happens when the cornea (clear dome that covers the front of the eye) is not curved enough or the length of the eye is too short. Distance objects are seen more clearly than near objects. Hyperopia can be corrected with glasses, contact lenses or in some cases, laser surgery.
What is astigmatism?
Astigmatism occurs when the cornea (clear dome that covers the front of the eye) and/or the lens inside the eye are irregular in shape. Light entering the eye does not converge into one point of focus on the retina (photosensitive nerve layer at the back of the eye). As a result, vision is blurred at all distances. Astigmatism can be corrected with glasses, contact lenses or laser surgery.
What is presbyopia?
To focus near objects on the retina (photosensitive nerve layer at the back of the eye), the lens in the eye changes shape with the use of focusing muscles. With age, the lens becomes stiff and the muscles are no longer able to change the shape of the lens. Presbyopia generally becomes a problem after age 40. Near objects are blurred. Presbyopia can be corrected with glasses and sometimes contact lenses.
What is strabismus?
Strabismus is commonly referred to as a “turned eye”. There are 6 extraocular muscles which move the eyes and keep the eyes aligned. When there is an imbalance in the extraocular muscles, strabismus results. The deviating eye can turn in (esotropia), out (exotropia), up (hypertropia) or down (hypotropia). Strabismus is treated with glasses, contact lenses or surgery. Strabismus is not the same as amblyopia (lazy eye) but strabismus can cause amblyopia.
What is amblyopia?Amblyopia is commonly referred to as a “lazy eye”. Amblyopia is reduced vision which is not correctable with glasses or contact lenses, in the absence of eye disease. Amblyopia occurs when the visual centres in the brain do not fully acknowledge the images seen by the amblyopic eye. This can occur with strabismus (turned eye) or when there is a large difference in prescription between the eyes, for example high hyperopia (farsightedness) in one eye. Amblyopia can also occur if the vision is obstructed in one eye, for example if there is a significant drooping of the upper lid. As a result, the visual pathways of the amblyopic eye do not develop normally and vision is decreased. Amblyopia almost always affects one eye. Treatments for amblyopia include eyeglasses, contact lenses, patching therapy, vision training and surgery.