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

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)

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

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 = 7 + 58 = 65 Minimum blank size would be 65

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

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