Besides common short-sightedness, there are also far-sightedness and astigmatism as well as various mixed forms of these visual defects. Apart from these forms of defective vision, there are also individual visual defects of a higher order - the so-called aberrations, which make seeing difficult, particular in the dark.

The medical term for short-sightedness is myopia.

The short-sighted eye is longer than the normal-sighted eye. The focal point of the incidence of the light rays thus does not lie directly on the retina, but rather in an area in front of it. Short-sightedness means that we can see clearly what is near, but things further away are unclear. With increasing age, people with slight short-sightedness (up to approx. -3.0 diopter) have the advantage that they can see things close up relatively well without glasses and often need reading glasses later or not at all.

The medical term for far-sightedness is hyperopia.

In comparison with the normal-sighted eye, the far-sighted eye is too short. The focal point of the incidence of the light rays thus does not lie directly on the retina, but rather in an area behind it. Far-sightedness means that we can see clearly in the distance, but not at a near range. With increasing age, near-vision capacity becomes even worse and reading glasses are needed at about age 45.

The medical term for optical defects due to corneal curvature is astigmatism.

In people with astigmatism, the shape of the cornea is not round. The imaging on the retina is thus not dot-formed, but in lines. This is where the name astigmatism comes from. Short or far-sightedness often occurs in association with astigmatism. As a result of astigmatism, vision is blurred both up close and in the distance.