More about the eyes: Optical Corrections
Myopia - This is often referred to as "short-sightedness" and is the condition where near objects are in focus and distant objects are blurred. The focal point of the eye is in front of the retina instead of directly on it. This makes distance vision blurry. The spectacle correction is with a "minus" powered lens e.g. -2.00DS. A minus powered lens is thicker at the edge and thinner in the centre and it makes objects seen through it appear smaller.
Hyperopia - Commonly referred to as "long-sightedness" this results in the inability to focus easily on objects close up. With this condition the image is focussed behind the retina instead of on it. For these people the effect varies depending on the strength of their prescription and age. The spectacle correction is with a "plus" powered lens e.g. +2.00DS. The lens is thicker in the centre and thinner at the edge. It makes objects seen through it appear bigger.
Presbyopia - As people get older, the crystalline lens which is positioned behind the iris cannot flex as easily and this inhibits the ability to focus on nearby objects. This is the reason why people with good vision when they are young require spectacles in middle or old age. The usual age of onset of this problem is forty to forty-five and it eventually happens to us all. The "near add" in your prescription is the extra power you require to focus for near vision. Reading glasses provide such extra power for reading a book or a computer screen but these will make distance vision blurry.
Astigmatism - Astigmatism occurs in all age groups and affects both distance and near vision, and may occur with myopia or hyperopia. It occurs when the cornea is not perfectly spherical, causing a portion of an object to be in focus and the remainder of the object to be out of focus. We use toric lenses to correct this defect.
Anisometropia – This is when there is a significant difference in the refractive state between the two eyes resulting in different sized imaged being formed on each retina, so when the brain attempts to put the two images together to get 3D vision, it is not able to do so. The person is likely to feel uncomfortable, not be able to wear their spectacles, and may have double vision. Contact lenses provide an excellent solution, as the optical correction will be much closer to the eye and the difference in the image on each retina much less.
Strabismus (Crossed Eyes/Squint) – Occurs when one eye turns in, out, up or down. This is often caused by poor eye muscle control. Treatments include eyeglasses, prisms, vision therapy, and sometimes, surgery. If detected early, it can often be corrected.
Amblyopia (Lazy Eye) – Loss or lack of vision development in one eye. Often associated with crossed-eyes or large difference in powers between the two eyes. Treatments include prescription lenses, prisms, vision therapy and eye patching.
Aphakia– This iswhen the lens inside the eye is absent, usually as a result of cataract surgery. The lens of the eye has strong optical power, so aphakics need to wear either strong spectacles (about +12D) or strong contact lenses. When the lens of the eye is removed with cataract surgery, nowadays an intra-ocular lens (see Cataracts, below) is usually inserted to provide optical correction.