ABO NOCE Practice Test

What formula gives the GCD or Frame PD?

1. GCD = A + DBL

2. GCD = DBL - A

3. GCD = A - DBL

4. GCD = A + PD

The correct answer is: GCD = A + DBL

The formula that correctly gives the GCD (Geometric Center Distance) or Frame PD (Pupillary Distance) is GCD = A + DBL. This formula incorporates two critical components: 1. "A" represents the distance from the geometric center of the face to the pupil. 2. "DBL" stands for Distance Between Lenses, which is the distance between the two optical centers of the lenses. By summing these two measurements, the formula effectively calculates the total distance that accounts for both the individual eye position and the separation necessary for proper lens placement. This results in an accurate GCD, which is vital for ensuring that lenses are centered correctly over the pupils, leading to optimal visual performance and comfort. The other formulas provided involve variations of addition and subtraction of these components but do not accurately represent the relationship needed to determine the GCD. Therefore, those options are not correct as they do not provide the necessary calculation to find the GCD or Frame PD.