What is the hardest curve to measure in contact lens fitting?

In the context of contact lens fitting, the hardest curve to measure is typically the posterior central curvature, often referred to with the abbreviation PCW. This curve is critical because it directly affects the lens's fit and performance on the cornea. The posterior central curvature must match the corneal shape accurately to ensure that the lens sits properly without causing discomfort or damage to the eye.

The challenge in measuring this curvature arises from a combination of factors. First, it requires precise instruments and techniques to ensure that measurements are taken correctly, as even minor discrepancies can lead to significant implications for lens fit. Additionally, the central portion of the cornea tends to be more variable in shape between different individuals, making standardization difficult.

While other curves in the fitting process, such as the base curve, peripheral curve, and intermediate curve, are also important, they tend to be easier to measure accurately due to established reference points and methodologies that are commonly used in practice. The posterior central curvature, however, demands a higher degree of precision and understanding of corneal topography, thus classifying it as the hardest curve to measure in the context of contact lens fitting.