In a manual lensometer, what happens if a cylinder is present in the lens?

When using a manual lensometer, the presence of a cylinder in the lens indicates that there is astigmatism correction. The lensometer measures various meridians of the lens, and if a cylinder is present, it will demonstrate different powers in different orientations.

The correct choice highlights that the difference in power between the sphere and cylinder lines on the lensometer scale reflects the amount of cylinder present in the prescription. This means that if the lensometer shows two distinct power readings in different meridians—one for the sphere and one for the cylinder—the numerical difference between these readings indicates the strength of the cylinder correction needed. This information is crucial for accurately determining the prescription for the patient and providing optimal optical correction.

In contrast, stating that the sphere power equals the cylinder power, or that the sphere and cylinder powers are additive, does not accurately describe the relationship between these elements in a prescription with astigmatism. Moreover, claiming that the cylinder power is always zero would negate the entire concept of astigmatism correction. Thus, understanding how cylinder power is represented and assessed is vital for anyone involved in eye care, particularly when refraction and lens customization are necessary.