What does the term "astigmatism" refer to?

Astigmatism refers to a refractive error primarily caused by an irregular shape of the cornea or lens. In a normal eye, the cornea and lens have a smooth, evenly curved surface that allows light to focus evenly on the retina. However, when there is an irregular shape, light rays are bent unequally, resulting in blurred or distorted vision at any distance. This condition can lead to difficulties in visual acuity, as objects may appear stretched or wavy, rather than sharp and clear.

The other choices do not accurately define astigmatism. The second choice describes myopia (nearsightedness), where the eye has trouble focusing on distant objects due to an elongated shape. The third choice refers to photophobia, which is an increased sensitivity to light, unrelated to the refractive errors caused by astigmatism. The fourth choice describes visual fatigue or eye strain, which can lead to temporary blurriness but is also not specifically related to the structural irregularities that define astigmatism.