When our visual acuity decreases
In order to see objects clearly, the incoming light must be focused on the retina. The better this process works, the sharper our vision - and we are then able to distinguish objects from each other with greater precision. Visual acuity is the sharpness with which an object is focused on the retina.
Reduced visual acuity
Visual acuity is influenced by a wide variety of factors, which are determined not only by the eye itself but also environmental influences. The interaction of the eye media such as cornea, lens or vitreous humour with the retina can lead to reduced visual acuity, for instance, if the media do not focus the incoming light on the retina, but instead focus it in front of the retina (short-sightedness), or behind it (long-sightedness), or if one focus point on the retina is not attained (astigmatism). In addition, damage to the eye media, the retina or the optic nerve can lead to reduced visual acuity.
Visual acuity in ophthalmology
However, determining visual acuity is also an absolute necessity when using magnifying vision aids. In order to correct vision defects, ophthalmologists and opticians need to determine the corrected visual acuity (lat. visus cum correctione) - i.e. your best corrected visual acuity. The determination of visual acuity is not only necessary for fitting glasses or contact lenses, but it also provides the basis for optimum selection and individual adjustment of magnifying vision aids.
Determining visual acuity
In order to determine the corrected visual acuity, a variety of test symbols are used by ophthalmologists and opticians. In addition to Landolt rings, Snellen charts, letters and numbers, symbols can also be used which are displayed at a specified distance via charts or test symbol projectors. Despite their differences in appearance, all these test symbols have one thing in common - their size is always five times the thickness of the line, and in the case of the Landolt ring, the gap width is also the same as the thickness of the line. During the sight test, if the person who is being tested can recognise a test symbol that has a line width of less than 1 minute of arc, then their visual acuity will be 1. If the person being tested has a visual acuity of only 0.5, then the test symbol displayed must be doubled in size in order for it to be recognised.