American Board of Ophthalmology (ABO) Practice Exam

If the patient's pupillary distance is greater than the frame PD, how is the decentration affected?

1. Decentration is in

2. Decentration is out

3. Decentration is neutral

4. Decentration is variable

The correct answer is: Decentration is out

When the patient's pupillary distance (PD) is greater than the frame PD, the lenses need to be positioned further away from the center of the optic zone to align with the patient's pupils. This misalignment creates a situation where the optical center of the lens is moved laterally away from the center of the frame, resulting in what is known as "outward decentration." In practical terms, when fitting lenses into frames, if the patient's PD exceeds the frame PD, the lenses will have to be decentered outward to properly align with the patient's pupils for optimal vision. This outward movement ensures that the visual axis coincides with the optical center of the lens, thereby minimizing visual distortion and maximizing clarity. Understanding decentration is vital in ophthalmology, as improper positioning of the lenses can lead to discomfort and visual disturbances, such as prismatic effects or blurred vision. This emphasizes the importance of accurately measuring both the patient's PD and the frame PD during the lens fitting process.