Which of the following defects of vision can be corrected by using cylindrical…

Concept: Defects of vision are refractive errors of the eye. Spherical errors (the eye's focusing power is uniformly too strong or too weak in every meridian/plane) are corrected with ordinary spherical (concave/convex or bifocal) lenses. Astigmatic errors occur when the cornea or the eye lens is unevenly curved, so its focusing power differs from one meridian to another; because a cylindrical lens has curving power confined to a single plane, it is used specifically to cancel this meridian-dependent (non-spherical) error.

Application: Among the four listed defects, only astigmatism is caused by this non-spherical, meridian-dependent curvature — the cornea or lens bends light more strongly along one plane than another, so incoming rays converge at more than one focal point instead of a single sharp point. A cylindrical lens's power acts in only one plane, so it exactly cancels the extra curvature in the affected meridian while leaving the other meridian's focusing power unchanged, restoring a single point of focus. Presbyopia, hypermetropia, and myopia are each a uniform (spherical) gain or loss of focusing power / accommodation range, so ordinary spherical or bifocal lenses — not cylindrical ones — correct them.

Contrast — each option by its own mechanism:

• Presbyopia: an age-related loss of elasticity in the eye lens uniformly reduces its power to accommodate for near objects; corrected with convex lenses for near vision (bifocal lenses when a distance correction is also needed).

• Hypermetropia: the eyeball is too short, or the lens's converging power is too weak, equally in every meridian, so near images form behind the retina; corrected with a uniformly converging convex spherical lens.

• Myopia: the eyeball is too long, or the lens's converging power is too strong, equally in every meridian, so distant images form in front of the retina; corrected with a uniformly diverging concave spherical lens.

• Astigmatism: the only one of the four with meridian-dependent (non-spherical) curvature, so a cylindrical lens is the mechanism-appropriate correction.

Result: The defect of vision corrected using cylindrical lenses is Astigmatism.