Emmetropia: An eye is called emmetrope or normal sighted if it can produce a sharp image from distant objects. All other conditions which differ from an emmetropia are so called refractive errors.
The most important refractive errors are Myopia, Hyperopia, Astigmatism and Presbyopia.
Myopia: Nearsightedness or Myopia is a refractive error when you can see the nearby objects more clear than the distant objects. In most cases it is because the eye is too long or because the refractive power of the eye is too strong. It appears often in the time of puberty and progresses till the age of about 25 and remains throughout the whole life.
Hyperopia: In the case of hyperopia or farsightedness the eye is too short or the refractive power of the eye is too weak. In the young age and depending on the degree of hyperopia one can still see clear in the distance and nearby as long as the accommodation or the ability to focus still exists. As we age we lose our ability to focus first the nearby objects and later the distant.
Astigmatism: Astigmatism causes blurring of images at all distances. It exists usually from the time of birth and can be associated with myopia or hyperopia. The higher the irregularity of the cornea the higher is the distortion of the image.
Presbyopia: Presbyopia is a normal part of the aging process and can affect everybody from the age of 40. As we age the natural lens in the eye loses its flexibility and becomes more dens so that the nearby objects can no longer be seen clearly. People usually compensate this by holding reading materials further away. Patients often complain that their arms are not long enough anymore. At this stage, most patients require reading glasses. In general presbyopia can be corrected with multifocal lens implantation or monovision. Monovision means that the dominant eye is corrected for the distant vision and the non dominant eye is corrected for near vision. In order to confirm that the patient tolerates monovision, this condition can be simulated with contact lenses.