What defect in vision is caused by an irregularity of the cornea?

Astigmatism is a defect in vision that results from an irregular shape of the cornea or sometimes the lens of the eye. In a normally shaped cornea, light rays entering the eye are focused evenly on the retina, allowing for clear vision. However, when the cornea has an asymmetrical shape, light rays are refracted unevenly, leading to blurred or distorted vision at various distances. This irregular curvature means that different meridians of the eye can have varying focal lengths, which is the hallmark characteristic of astigmatism.

In contrast, myopia (nearsightedness) occurs when the eyeball is too long or the cornea is too steep, hyperopia (farsightedness) happens when the eyeball is too short or the cornea is too flat, and presbyopia is related to the aging of the lens, leading to difficulty in focusing on close objects. Each of these conditions has different underlying causes related to the shape of the eye or the lens but does not specifically stem from corneal irregularities like astigmatism does.