What common visual defect does astigmatism cause?

Astigmatism is a refractive error that occurs when the cornea (the clear front surface of the eye) is not perfectly spherical, leading to uneven focusing of light on the retina. This uneven curvature can cause light rays to be focused at different points, resulting in blurred or distorted vision. Individuals with astigmatism often report that straight lines may appear wavy or that their vision may seem out of focus, regardless of the distance of the object being viewed.

This characteristic cause of blurred or distorted vision is the hallmark symptom of astigmatism, making it the primary visual defect associated with this condition. The other options such as double vision, loss of peripheral vision, and night blindness can be attributed to different eye conditions or diseases but are not directly related to the nature of astigmatism itself.