Astigmatism
The cornea is the anterior, foremost delimitation of the eye. In most humans, the cornea does not show a perfectly spherical curvature, but rather looks like an egg, with a strong curvature in one axis, and a weaker curvature perpendicularly to it. Therefore, the optics does not display a focal point, but rather a focal line. In healthy eyes, the axes are regular to each other and glasses allow for a good visual acuity.
In rare cases, the differences in the curvature are rather irregular, and the cornea “looks “like a landscape, with hills and valleys”. Such an irregular astigmatism can be seen after accidents with scar formation and in diseases like keratoconus. Glasses can only partially help to increase vision.
