1. Decentration & Minimum Blank Size
California State Society for Opticians
January 29, 2017

2. Decentration
This is the amount of movement created by the patient’s pupil distance (PD’s) and frame optical center.
Decentration Formula
A size + DBL – Patient’s PD’s = Decentration/2 (per eye)

3. Calculate Decentration
Answers
Frame A: 54, B: 34 DBL: 18, ED: 55
Patient PD’s = 62
Frame A: 52, B: 35 DBL: 16, ED: 55
Patient PD’s = 59
Frame A: 51, B: 36 DBL: 17, ED: 55
Patient PD’s = 65
Frame A: 55, B: 32 DBL: 16, ED: 56
Patient PD’s = 60
Frame A: 52, B: 32 DBL: 18, ED: 55
Frame A: 54, B: 30 DBL: 15, ED: 55
Patient PD’s = 58

4. Minimum Blank Size The formula for determining the smallest
possible lens blank which will work for any
given frame and PD combination is as follows.
Minimum Blank Size (MBS)
Geometric Center Distance GCD or frame PD -
Pupil Distance PD) + (Effective Diameter ED)
or -
MBS = (GCD – PD) + ED
Example: Frame Size 50-20, PD 63, ED 58
70 – 63 = = 65
Minimum blank size would be 65

5. Calculate Minimum Blank Size
Answers
Frame A: 54, B: 34 DBL: 18, ED: 55
Patient PD’s = 62
Frame A: 52, B: 35 DBL: 16, ED: 55
Patient PD’s = 59
Frame A: 51, B: 36 DBL: 17, ED: 55
Patient PD’s = 65
Frame A: 55, B: 32 DBL: 16, ED: 56
Patient PD’s = 60
Frame A: 52, B: 32 DBL: 18, ED: 55
Frame A: 54, B: 30 DBL: 15, ED: 55
Patient PD’s = 58

6. Extra Work
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