Which refractive error occurs when light rays enter the eye unevenly?

Astigmatism occurs when light rays enter the eye unevenly due to an irregular shape of the cornea or lens. In a normally functioning eye, light rays focus evenly on the retina, leading to clear vision. However, in cases of astigmatism, the curved surface of the eye is not symmetric, causing light rays to focus at different points either in front of or behind the retina. This results in distorted or blurred vision at any distance.

Different refractive errors affect vision in distinct ways. Myopia (nearsightedness) leads to distant objects appearing blurry because light focuses in front of the retina. Hyperopia (farsightedness) causes nearby objects to seem blurry as light focuses behind the retina. Presbyopia is an age-related condition where the eye's ability to focus on nearby objects diminishes over time. While these conditions impact vision, they do not specifically result from uneven entry of light rays into the eye, which is characteristic of astigmatism.