Visual dioptric defects
Healthy eye
The eye clearly visualizes faraway objects in infinite distance as a point on the retina, without any dioptric error , i.e. the focus lies exactly on the retina. The length of the eye (approx. 24 mm), when looking at distant objects corresponds to the eye refractive power (D = +59dpt). A distant point lies in infinity.
Emetropic eye
Shortsightedness – myopia
The shortsightedness is characteristic for its prolonged front–rear axis
(1 mm prolongation is about 3 dioptres) and/or there is a big refractivity of
the optical system. Its causes are hereditary factors, excessive eye stain in
adolescence and others. Shortsightedness is a very frequented defect, the past
years have seen a very high growth of this defects. For example, in the USA the
number was 14% of population in 1981 and this soared up to 25% in the last
year. A short-sighed person usually sees well close objects and badly faraway
objects. 90% of short-sighted persons have less than –6 dioptres, 5% –6
to –10 dioptres and 5% –10 to –30 dioptres. A myopic eye focused at
distant objects has its focus positioned inside the eye before the retina. As a
result this is a cause of decreased sharpness when looking at distant objects.
Myopic eye
Farsightedness – hypermetropia
The shortsightedness is characteristic for its short front–rear axis and/or there is a small refractivity of the optical system. Its values usually vary between +2 to +6 dioptres. Almost 35% of the population suffers from farsightedness. A farsighted person may have just slight vision problems in youth but as the person grows older problem with both near and far vision appear. Quite common is the simultaneous occurrence of cross-eye (strabismus) or lazy-eye blindness (amlyopia).
A hyperopic eye focused at distant objects has its focus positioned outside the eye behind the retina. The Farsightedness can be completely or partially removed by increasing of the refraction of the eye (accommodation). Therefore a hyperopic eye can see with the help of accommodation distant objects clearly.
Hypemetropic eye without accommodation
Astigmatizms
Sometimes occurs separately, but often in conjunction with shortsightedness
or farsightedness. It is a cylindrical error. In mutually perpendicular axes the
eye has a different dioptric power. Consequently, the eye is not of spherical
shape, but slightly that of a deflated ball. A viewed object is not projected
sharp-edged, eg. a square is, figuratively speaking, perceived as an oblong or
rhomboid. A point is not perceived as a point, but as two lines. An astigmatic
defect significantly worsens the quality of vision. 
Astigmatizms
Purblindness
Purblindness is not a dioptric defect, but functional one when the visual analyzer in the brain suppresses an image perceived by the eye with a higher number of dioptres. According to current medical science, purblindness can be cured in childhood (up to about 9 years). However, we can often see improvement following a laser correction.
Presbyopia
Presbyopia is not a classic dioptric defect. This is a physiological loss of accommodation, consequently the capability of focusing on close objects.
Accommodation increases the curvature of the ocular lens, mainly its front surface. With the coming age the ocular lens elasticity decreases and with it the capability of accommodating.
Even if this process begins as early as in youth, the defect becomes more significantly apparent only in the age of about 40 years, when our capability of accommodation drops to such a degree that the eye is unable to focus on close objects. In order to read a newspaper or a book, we must hold them at an arm’s length – and soon even the arm cannot stretch far enough!
With the coming age the situation worsens and stabilizes at the age of about
60. Spectacles for close vision are used by 40% of middle-aged people and 90%
of higher age. 
Presbyopic eye focused on close objects.
Horizontal cross-section through the human eye
