Synonyms in a broader sense
Visual acuity, visual acuity Long-sightedness, short-sightedness, astigmatism, low vision
The examination of visual acuity is usually carried out by an ophthalmologist, but also by non-medical personnel, such as opticians or orthoptists, or other medical personnel by means of an eye test. Visual acuity is always measured separately for each eye. Thus, one eye is covered while the visual acuity is determined for the other.
If the visual acuity (measure of the vision of the eye) of both eyes is measured at the same time, the value would be distorted and there would be no possibility to determine a possibly worse seeing eye. Visual acuity is the ability of the eye to perceive two points separately. In this process, visual weaknesses may be corrected.
For the examination of visual acuity, standardized visual sample charts are available, which have proven to be particularly representative. On the one hand there are boards with so-called Snellen hooks. Here the patient sees the capital letter E, whereby the three horizontal lines normally pointing to the right can point in all 4 directions (top, bottom, right and left).
The patient must recognize this direction. There are also vision panels with rings that have an opening whose direction must be indicated. There are also boards with numbers, letters or – especially for children – with objects (horse, cup, scissors etc.).
From top to bottom the symbols become smaller and smaller. Next to the lines is the distance at which a normal sighted person with visual acuity 1.0 can read them. Next to the line with the largest characters is <50m>. So this line can be read by a normal sighted person without visual impairment from a distance of 50 meters without problems. These values serve as an orientation of the severity of the present visual impairment.
Visual acuity testing
The vision panel is placed at a distance of 5 meters from the patient. If the patient wears glasses, the test is performed first without, then with glasses. Vision without glasses is also called raw visual acuity or visual performance, with glasses it is called visual acuity or visual acuity.
The visual acuity is also expressed as a break. This is where the numbers that provide information about the reading distance of a normal sighted person are used. The nominal distance, the line just to be read, is in the denominator.
If the patient can only read the top line from the given distance of 5 meters, the fraction is 5/50. The patient has a low visual acuity. For example, a visual acuity of 1.0 is 5/5.
At 5/4 the visual acuity is even higher than 1.0. In this case, the patient can read the last line, which should actually be read from a distance of 4 meters, from a distance of 5 meters. However, there are also people who cannot see even the top line from a distance of 5 meters.
In this case the board is shown at a distance of 1 meter. The break must then be changed accordingly. In the counter (i.e. at the top) the 1 is now written, not the 5th. If the patient cannot even overcome the distance of 1 meter, other aids are used. Finger counting, hand movement or, in the worst case, light projection are the most common ones.
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