Using the data of optic values, it is possible to calculate refractive force of optic eye system, which is on the average from 58.0 to 60.0 D, but it may oscillate from 52 up to 80 D. It is a physical refraction of the eye. However, in clinic the position of main focus 20 concerning to retina has significance for refractive force of eye environments. This determines concept of clinical refraction — correlation between physical refraction and length of eye axis.
Three versions are possible: the main focus is placed on a retina, before it, or behind it. In this connection, there are three kinds of clinical refraction. The first one when the parallel rays, having refracted, incorporate on a retina; it is a proportional refraction — emmetropia. When harmony is not present, ametropia takes place (nearsightedness or hyperopia). If the parallel rays, having refracted, incorporate in main focus in front of retina, it is nearsightedness (myopia, strong clinical refraction). When the parallel rays having refracted incorporate, behind a retina, the hyperopia (hypermetropia, weak clinical refraction) is watched.
Thus, the kinds of a clinical refraction are characterized by a position of a main focus concerning to retina.
Ametropia can depend on that the diopter apparatus refracts stronger or weaker comparing with the norm (refractive ametropia), or on that longer or shorter than normal the axis of the eye (axial ametropia).
The kinds of clinical refraction are characterized also by attitude to rays of different direction.
At emmetropia the parallel rays incorporate on the retina, i.e. the rays, which go from infinity. In myopic eye the divergent rays going from distance closer than 5 m are focused on the retina. At hyperopia theoretically only the convergent rays could collect on a retina which do not exist in the nature.
The point, the most remote from the eye, which is visible at accommodation rest is named as a further point of clear vision (punctum remotum). In emmetropia it is infinity, in myopia it is closer than 5 m, and in hypermetropia it does not exist.
Development of the refraction. The most part of the children are born with a strong physical refraction (80 D). However, owing to that length of eye axis is small (18.5 mm), clinical refraction of such an eye is weak (hypermetropia — about 4.0 D). To 3 years the physical refraction makes up 60 D, practicall> without changing in further and the clinical refraction becomes more stronger averaging +2.0 D to 3^1 years, + 1.0 D to 6-8 years and after 9-12 years there is emmetropia. The length of eye axis in these periods is accordingly 23.5; 23.7 and 24 mm.
Ways of Definition of Clinical Refraction
There are subjective and objective methods of clinical refraction definition. The method based on selection of corrective lens concerns to a subjective method. By the kind of lenses, with which one eye can see better, it is possible to define the character of refraction (myopia or hypermetropia), and the force of these lenses determines a degree of anomaly. The objective methods consist of direct ophthalmoscopy, shadow test — skiascopy, refractometry (on Hartinger refractometer and dioptrone).
For children and young people the refraction is determined only by objective methods after cycloplegia (atropinization).