**Core Concept**
Keratometry is a diagnostic technique used to measure the curvature of the cornea, which is essential for calculating the power of the cornea in the context of refractive errors. This measurement is crucial in ophthalmology and optometry to assess the cornea's contribution to the eye's total refractive power.

**Why the Correct Answer is Right**
Keratometry measures the curvature of the cornea using a keratometer, which projects a series of light rings onto the cornea. The reflected rings are then used to calculate the corneal curvature, taking into account the anterior and posterior corneal surfaces. This information is vital for determining the correct lens power for corrective lenses, such as glasses or contact lenses, and for assessing corneal astigmatism.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because keratometry is not primarily used to assess the clarity of vision or visual acuity.
**Option B:** This option is incorrect because corneal topography is a more advanced technique used to map the surface of the cornea in greater detail, rather than just measuring its curvature.
**Option C:** This option is incorrect because tonometry is a technique used to measure intraocular pressure, not corneal curvature.

**Clinical Pearl / High-Yield Fact**
A normal corneal curvature is typically measured between 38-45 diopters (D), with an average value of around 43 D. This value can vary slightly between individuals, but significant deviations can indicate corneal astigmatism or other refractive errors.

**Correct Answer: C.**