What tool is primarily used to measure the curvature of the cornea?

The keratometer is the primary tool for measuring the curvature of the cornea. This device provides precise measurements of the anterior surface of the cornea, which is crucial for fitting contact lenses and assessing corneal health. By determining the radius of curvature, the keratometer helps clinicians evaluate astigmatism and ensure that lenses fit appropriately on the eye's surface, which is vital for both vision correction and comfort.

Understanding the function of different instruments is key in clinical practice; while an otoscope is used for examining the ear canal and tympanic membrane, and a tonometer measures intraocular pressure to assess conditions such as glaucoma, neither of these tools is designed for corneal curvature measurement. Similarly, a retinoscope is used to assess refractive error by evaluating the reflection of light off the retina, but it does not measure corneal curvature. Thus, recognizing the specific application of the keratometer in determining corneal shape distinguishes it as the correct choice for this question.