Which condition is characterized by having two different curves on a single refracting surface?

Astigmatism is characterized by having two different curves on a single refracting surface. This condition occurs when the cornea or lens has an irregular shape, leading to a refractive error where light rays entering the eye do not focus evenly on the retina. Instead, some rays are focused at one point while others are not, which can cause blurred or distorted vision at various distances. The presence of two different curvatures allows for different focal points, which creates the characteristic symptoms associated with astigmatism, such as difficulty seeing clearly at both near and far distances.

In contrast, the other conditions mentioned do not involve this specific characteristic of differing curvatures on a single refracting surface. Anisometropia refers to a significant difference in the refractive power between the two eyes. Presbyopia is related to the natural aging process affecting the ability to focus on near objects due to a decrease in the elasticity of the lens. Myopia, or nearsightedness, occurs when the eye is too long or the cornea has too much curvature, causing distant objects to be blurry, but it does not involve the presence of two different curves on the same refracting surface like astigmatism does.