What defines astigmatism?

Astigmatism is specifically characterized as a refractive error that occurs due to an irregular shape of the cornea or lens, leading to light rays being focused at different points. This results in two focal points rather than a single one, causing blurred or distorted vision. Patients with astigmatism often experience difficulty seeing objects clearly at any distance due to this dual focal point issue.

The other options describe different visual conditions. For example, a condition with equal focal points for light rays refers to normal vision or emmetropia, where light is focused at a single point on the retina. The idea of a total inability to focus light on the retina pertains more to conditions like retinal detachment or severe refractive errors rather than astigmatism. Lastly, age-related vision changes typically refer to presbyopia, which involves a decreased ability to focus on close objects, rather than the dual focus aspect that defines astigmatism.