There is a distinction between physical and clinical refraction of the eye.
Physical refraction is the refractive power of the optical system of the eye. It consists of the cornea, lens, watery moisture and vitreous body. The strength of the optical system is measured in diopters. It is this unit of measurement of refraction that is used in the appointment of corrective glasses and contact lenses.
Clinical refraction of the eye is the ratio of the focal length of the refractive power with the distance from the anterior surface of the cornea to the retina – the length of the anteroposterior axis of the eye. Normally, they should match.
Depending on the ratio of the focal length of the optical system to the length of the anteroposterior axis of the eye, there are 2 types of refraction:

• Proportionate (emmetropia). The refracted rays end up on the retina – the length of the focal length of the refractive system coincides with the length of the anteroposterior axis of the eye. Refraction is normal – the patient sees objects clearly.
• Disproportionate (ametropia). The refracted light rays end up in front of or behind the retina – the focal length of the refractive system does not match the length of the anteroposterior axis of the eye. Refraction is impaired – the patient sees objects indistinctly.

Types of refractive errors:

• Hypermetropia (hyperopia). The refracted rays of light gather behind the retina of the eye. The patient sees blurrily in the vicinity, and with a high degree of pathology, in the distance.
• Myopia (nearsightedness). The refracted rays of light gather in front of the retina. The patient sees clearly close objects and blurrily distant ones.
• Astigmatism. The presence of several focal points in one eye. The patient sees the images distorted.

Several research methods are used to determine the degree of refraction. They are divided into objective and subjective ones. The objective ones include:

• Skiascopy. The doctor observes the movement of the shadow that forms when the light beam hits the pupil. Depending on the location of the shadow, the specialist diagnoses a specific refractive error.
• Refractometry. Using an autoreferatometer, the doctor determines the refractive power of the eye. The method is considered the most accurate.
• Subjective methods for determining refraction include visual acuity testing tables and a set of optical glasses with test frames.

To correct visual acuity, the patient is offered glasses or contact lenses.
Eyeglass vision correction is the most common method. The patient is prescribed monofocal glasses with diopters of "plus" or "minus" depending on the refractive error – farsightedness or myopia.
With the development of presbyopia, the doctor suggests other options for glasses for correction.:

• Bifocals – with 2 optical zones, clearly delimited zones for proximity and distance.
• Office – with 2 optical zones with smooth transitions for viewing objects in the vicinity and at an average distance.
• Progressive – with 3 segments: the patient clearly sees objects at long, medium and short range.

For astigmatism, individual glasses with complex lenses are made, which contain a spherical and cylindrical component.
When choosing complex glasses to correct presbyopia or astigmatism, the doctor gives the necessary recommendations for adapting to their wearing.
If the glasses do not fit, the specialist selects contact lenses. To correct refractive errors, the eye may be prescribed:

• Сферические линзы – solve problems of first and second degree myopia and hyperopia.
• Aspherical lenses correct all forms of vision impairment. They are lighter and thinner than spherical lenses and have less image distortion at the edges of the lens.
• Торические (астигматические) линзы – suitable for the correction of astigmatism.
• Мультифокальные линзы – equipped with three optical zones, which is suitable for correcting presbyopia.