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What is commonly called a defect of vision is referred to by specialists as a defect of refraction or an irregularity of the eye, or ametropia. When the eyes – or, professionally speaking, the optical system at rest – fail to focus the incoming, parallel beam of light on the retina, an ophthalmologist or optometrist diagnoses a visual defect: hyperopia, myopia and/or astigmatism (incongruity). Examination of refractive defect is necessary to select corrective glasses or corrective contact lenses and also to schedule (program) laser correction surgery.

In addition to the now common autorefractometer (autorefraction) test commonly referred to as a “computer vision test,” a subjective method of testing refractive error called the Donders method is still invariably used in the process of selecting eyeglass/lens correction and in the process of designing laser vision correction. This is the stage of the examination at which the ophthalmologist puts on our nose the characteristic eyeglass frames into which lenses of different powers are inserted, or uses a slightly more complicated device, i.e. a phoropter equipped with a magazine of lenses, which – thanks to a system of dials – can be easily and quickly set in the right combination (sphere, cylinder) in front of the examined eye.

In addition, it is possible to set up a crossed cylinder, a prism with variable power and base direction, a color or polarizing filter, as well as other auxiliary elements (e.g., a stenopic aperture, a Maddox stick). The lenses are changed and adjusted (either mechanically or automatically) until the person being tested says he or she finally sees optimally.

Study

During the examination, the doctor is constantly asking the patient about his subjective sensations and trying to determine how much and how contrasty he sees with the corrected eye when a particular lens is in the frame. The patient should be confident that the corrected eye sees as well as it can to any distance. The ophthalmologist, of course, has the option of reaching for tools (tests) to minimize the risk of confusion and check whether the patient sees adequately for his visual abilities. The presence of eye diseases, high myopia or hyperopia prevent full (100%, 1.0, 6/6, 20/20) visual acuity and the patient is informed of this fact during the examination. This test may be unreliable in children whose eyes have high accommodative capacity. It should also not be performed in adolescents or adults whose accommodations are very strong or have contracted. In the case of myopia, the refractive defect is considered to be the value (power) of the weakest distracting (minus) lens at which normal visual acuity is achieved, in the case of hyperopia it is the value of the strongest focusing (plus) lens. The autorefractometer test alone should not be the basis for a prescription for corrective glasses, as it can lead to the eye rejecting the correction altogether: there may be a sensation of a slight distortion of space (especially visible under the feet in the form of a sloping floor) or a floating image, as well as dizziness and headaches, eye pain and even nausea, which do not pass after a few days – and that’s how long the brain needs to get used to new glasses. If the discomfort persists, it means that the glasses were selected incorrectly. Based on the “computer printout” data alone, it is not possible to prescribe, much less make glasses or apply contact lenses. The most important thing is a matter of individual perception. It may turn out that the theoretical/calculated correction does not fully improve the patient’s visual acuity. Subjective perception of the environment varies from patient to patient, so results from mechanical devices cannot be directly translated into prescription. A subjective refraction test is essential.