Final answer:
A keratometer measures PD based on pupil center distances by moving the hairline to the pupil's center. Smaller image magnifications relate to a smaller radius of curvature of the cornea, critical for fitting contact lenses.
Step-by-step explanation:
It is possible to use a corneal-reflection-style instrument to measure a PD based on pupil center distances. To do this, the hairline within the device is moved to the center of the pupil rather than the center of the corneal reflection. Using the keratometer, an optometrist determines the curvature of the cornea, which is essential for fitting contact lenses properly. A small magnification indicates a smaller radius of curvature. When light reflects from the cornea, which acts as a convex mirror, the keratometer calculates the image magnification. For example, if an object's height is 1.50 cm and it's placed 3.00 cm from the cornea, resulting in an image 0.167 cm high, the magnification would be the ratio of the image height to the object height.
To find the radius of curvature, this magnification can be used along with the object distance and image distance. The radius of curvature is twice the focal length of the convex mirror formed by the cornea. In practice, calculating these parameters aids optometrists in ensuring that contact lenses fit the unique shape of a patient's cornea, contributing to clear and comfortable vision.