American Board of Ophthalmology (ABO) Practice Exam

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When a patient's pupillary distance is less than the frame PD, what conclusion can be drawn about the decentration?

Decentration is neutral
Decentration is out
Decentration is in
Decentration is variable

The correct answer is: Decentration is in

When a patient's pupillary distance (PD) is less than the frame PD, it indicates that the optical centers of the lenses are positioned more medially in relation to the patient's pupils. This means that the patient's eyes are not aligned with the center of the lenses, resulting in what is known as "inward decentration." In detail, when the frame PD is greater than the patient's PD, the lenses must be decentered inward to place the optical centers directly in front of the patient's pupils. This inward adjustment is necessary to ensure proper visual alignment and optimal vision through the lenses, avoiding potential distortion or prismatic effects that can arise from improper alignment. Consequently, the conclusion drawn from the situation where the patient's PD is less than the frame PD is that decentration is occurring inward, confirming the choice related to inward decentration.