Which of the following instruments is most commonly used to measure the diameter of the optical zone of a contact lens?

The measuring magnifier is particularly suited for determining the diameter of the optical zone of a contact lens because it provides a clear, magnified view of the lens, allowing for precise measurement. This tool can effectively assess the size of the optical zone directly on the lens surface by providing a visual aid to ensure accurate readings.

In practice, optometrists and contact lens fitters rely on this instrument to verify the dimensions of lenses, ensuring they meet specific parameters for optimal fit and performance. The clarity and magnification capabilities make it easier to differentiate between the various zones of the contact lens.

Other tools like the contactometer, radiuscope, and lensometer serve different functions. The contactometer measures the curvature of the lens surface, primarily assessing its fit and comfort; the radiuscope is used to measure the base curve of a lens, which is crucial for fitting but not specifically for the optical zone diameter; and the lensometer is designed for measuring the optical power of the lens and verifying the prescription. While these tools are valuable in their respective roles, they do not focus on measuring the diameter of the optical zone as directly or effectively as the measuring magnifier does.