Astigmatism means that the cornea has two curved planes, on radius is a steeper curve and one radius is a flatter curve. So the shape is oval like a football instead of spherical like a basketball. This causes light to focus on more than one point in the eye, resulting in blurred vision at distance or near. This difference in focus length is often described as seeing a “ghost image” around objects. Astigmatism often occurs along with myopia or hyperopia.
A cataract is an opacity that develops in the crystilline lens of the eye. Early on in the development of age-related cataract the power of the crystalline lens may be increased, causing near-sightedness or myopia, and the gradual yellowing and opacification of the lens may reduce the perception of blue colors. Cataracts typically progress very slowly and the change in visual acuity is gradual. The rate at which a cataract progresses can be enhance by trauma to the eye or certain medications, namely steroid use.
People with a family history of glaucoma have about a six percent chance of developing glaucoma. People of African-American and Asian decent are more likely to develop primary open angle glaucoma. Women are three times more likely than men to develop acute angle-closure glaucoma due to their shallower anterior chambers.
Those at risk for glaucoma are advised to have a dilated eye exam at least once a year.
Screening for glaucoma is usually performed as part of a standard eye exam performed by an optometrist or an ophthalmologist. Testing for glaucoma should include measurements of the intraocular pressure via tonometry, changes in size or shape of the eye, and an examination of the optic nerve head to look for any visible damage to it. If there is any suspicion of damage to the optic nerve, a visual field test should be performed.
Although intraocular pressure is only one major risk factors of glaucoma, lowering it via medications or surgery is currently the mainstay of glaucoma treatment.
Hyperopia, also known as farsightedness , is a visual defect caused by the eye being too short or when the lens cannot become round enough. This causes an inability to focus on near objects, and in extreme cases leaves a patient unable to focus on objects at any distance. As an object moves towards the eye, the eye must increase its power to keep the image on the retina. If the power of the cornea and lens is insufficient, as in hyperopia, the image will appear blurred.
The top picture is how the eye focuses with hyperopia. The bottom picture is how the eye focuses with corrective lenses for hyperopia.
Myopia also called near-sightedness, is a visual defect of the eye in which the eye focuses short of the retina (the back layer of the eye). Those with myopia see nearby objects clearly but distant objects appear out of focus. With myopia, the eyeball is longer than average, or the cornea is steeper than average, so images are focused in the vitreous inside the eye rather than on the retina.
The top picture is where the eye would focus with uncorrected myopia. The bottom picture shows how correction with glasses moves the focal point to the back of the eye.
Presbyopia is the diminished ability to focus up close, that occurs with aging. The most common theory is that it arises from the loss of elasticity of the crystalline lens.
Presbyopia is a condition that affects everyone at a certain age. The first symptoms are usually noticed between the ages of 40-50, though in fact the ability to focus declines throughout life. For those with good distance vision, it may start with difficulty reading fine print, particularly if the lighting is poor, or with eye fatigue when reading for long periods. Patients also feel like that are playing a trombone when trying to find a good distance at which to hold their paperwork to be able to read.
Presbyopia becomes much less noticeable in bright sunlight. This is not the result of any mysterious 'healing effect' but just the consequence of the iris closing to a pinhole which condenses the light to enhance focusing.