Ametropias
Astigmatism
Astigmatism is a very common refractive error that results, depending on its degree, in blurred vision at both near and far distances. Astigmatism is mainly due to the construction of the cornea of the eye which is not spherical. Astigmatism can also be due to the construction of the crystalline lens of the eye. This has the effect that the light rays do not converge at one point but on two different axes. Thus the image formed on the retina is blurred in both near and far vision.
There are two types of astigmatism, normal and abnormal. When we say normal astigmatism we mean that the cornea has an oval shape and can be corrected with glasses almost perfectly, while when we say abnormal astigmatism we mean that the cornea has a shape that does not allow it to be completely corrected with glasses. One such case is the condition called keratoconus.
Myopia
Myopia is a refractive error of the eye in which the rays of light are not focused on the retina, as is normal, but at some point in front of it. For this reason, the myopic (μύωψ in ancient Greek) is unable to clearly see the objects that are far away and half closes the eyes to do it better.
Hypermetropia
Hypermetropia, also known as hyperopia, is an eye condition in which light is focused behind, rather than above, the retina. This results in nearby objects appearing blurry while distant objects may appear normal. As the condition worsens, objects at all distances may appear blurred.
Presbyopia
Presbyopia is a vision disorder that usually appears at the age of 40-50 years and increases as we age. It is the reduction of the eye's ability to focus on close objects, as the normal aging process reduces the elasticity of the eye's lens.
Symptoms of presbyopia include difficulty focusing on close objects, eye strain, and headaches.
Presbyopia can be corrected with the use of special glasses or contact lenses.