Which part of the cornea does the keratometer primarily measure?

The keratometer is specifically designed to measure the curvature of the central part of the cornea, which is typically the central 3-4 mm area. This region is crucial for determining the refractive power of the cornea, which plays a significant role in how light is focused onto the retina.

By focusing on this specific area, the keratometer provides important data for designing contact lenses, assessing astigmatism, and diagnosing various corneal conditions. The central 3-4 mm of the cornea is where the majority of the corneal power is concentrated, thus making it a key area for accurate measurements.

The other options refer to either smaller or larger areas that do not conform to what the keratometer's design is intended to measure. For instance, measuring only the central 1-2 mm would not encompass enough of the cornea to provide reliable data, while measuring the peripheral regions or the entire cornea would be outside the typical application of the keratometer.