Minus cylinder lenses are prescribed because of
**Core Concept**
Minus cylinder lenses are a type of prescription eyewear used to correct astigmatism, a common refractive error caused by irregularities in the shape of the cornea or lens inside the eye. This results in blurred or distorted vision at all distances due to the unequal refractive powers in different meridians.
**Why the Correct Answer is Right**
The correct answer is related to the compensation of astigmatism, where the minus cylinder lens corrects the irregular curvature of the cornea or lens by introducing a complementary astigmatic refractive power. This is achieved by using a cylindrical surface with a minus power that counteracts the effect of the irregular astigmatism, resulting in clear vision. The minus cylinder lens is typically prescribed in combination with a spherical component to correct myopia or hyperopia.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because minus cylinder lenses are specifically designed to correct astigmatism, not myopia or hyperopia.
**Option B:** This option is incorrect because minus cylinder lenses do not correct presbyopia, which is a age-related loss of near vision due to decreased accommodation.
**Clinical Pearl / High-Yield Fact**
To remember the purpose of minus cylinder lenses, recall that "minus cylinder" sounds like "minus curvature," reflecting the lens's ability to correct irregular curvatures of the cornea or lens. This simple mnemonic can help students associate minus cylinder lenses with astigmatism correction.
**Correct Answer: C. Astigmatism is a refractive error caused by irregular corneal or lens curvature.**