What condition describes the curvature of the lens that does not refract light equally in all directions?

Astigmatism is characterized by an irregular curvature of the cornea or lens that prevents light from focusing evenly on the retina. In a normally shaped lens, light is refracted equally in all directions, allowing for clear vision. However, in astigmatism, the lens has different curvatures in different meridians, leading to varying focal points for each direction of light. This results in blurred or distorted vision, affecting the clarity of both near and far objects.

The other conditions listed represent different refractive errors or age-related changes in vision but do not describe the specific issue of unequal light refraction associated with the curvature of the lens. Hyperopia, myopia, and presbyopia are primarily related to the distance at which objects appear clear, rather than the irregular shape of the lens itself.