Final answer:
Astigmatism is a common vision defect that causes an unevenness or asymmetry in the focus of the eye. It can be detected using a chart, and it can be corrected with the use of eyeglasses or contact lenses.
Step-by-step explanation:
Astigmatism is a common vision defect that causes an unevenness or asymmetry in the focus of the eye. It occurs when the rays passing through different regions of the eye focus at different distances, resulting in distorted or blurred vision. This is primarily due to irregularities in the shape of the cornea, but it can also be caused by lens irregularities or unevenness in the retina.
Astigmatism can be detected using a chart, like the one shown in Figure 26.8. If lines along certain axes appear clearer or darker than others, it indicates the presence of astigmatism.
Astigmatism can be corrected with the use of eyeglasses or contact lenses. A cylindrical correction is prescribed to correct astigmatism, which compensates for the unevenness in the focus of the eye. Unlike spherical corrections used for short- or farsightedness, cylindrical corrections are specific to astigmatism and correct vision along different axes.