The dioptric power of the cornea is determined by:

The dioptric power of the cornea is primarily determined by keratometer readings. A keratometer measures the curvature of the anterior surface of the cornea, which is crucial because the shape of the cornea directly influences its refractive power. The steeper the cornea, the greater its ability to bend light, resulting in a higher dioptric power.

This measurement is essential for assessing lens prescriptions, planning refractive surgeries, and understanding various visual conditions. Accurate keratometric readings provide insight into how the light will be focused onto the retina, thus playing a critical role in optometry and ophthalmology.

Other methods listed, such as ultrasound readings, tonometer readings, and tomography readings, serve different purposes. Ultrasound can measure corneal thickness, tonometry is used to gauge intraocular pressure, and tomography provides detailed imaging of the corneal structure. However, these methods do not directly assess the cornea's curvature or its dioptric power like keratometry does.