Understanding Decentration

 Decentration is the displacement of a lens’s optical center from the patient’s visual axis. There are two key concepts, Vertical and Horizontal Decentration. Vertical decentration, commonly called Seg Drop,  is defined as the offset of a bifocal lens’s reading zone below the center of the frame.  Lenses may require vertical decentration (upward or downward movement) to match the patient’s OC (optical center) height, and may require horizontal decentration (right to left movement) to match the patient’s pupillary distance.

• Optical Center (OC) Height: The OC height is the distance from the bottom of the lens opening to the center of the patient’s pupil.

• Segment (Seg) Height: The Seg Height is the a measurement taken vertically from bottom of the lens to the top of the segment on a multifocal.

• Cylinder and Axis: When a lens prescription includes cylinder and axis for astigmatism correction, the orientation of the lens is crucial. It must be kept in the correct position to maintain the prescribed axis. This ensures the wearer can comfortably look down and see clearly through the bifocal area when reading.

• Pupillary Distance (PD): The PD is the distance from one pupil to the other.

Why it Matters: Proper decentration is crucial to avoid induced prism, ensuring comfortable vision and accurate optical correction. Misaligned lenses can strain the eyes and cause headaches.

Horizontal Decentration Calculation Formula:

• Binocular = (FPD – PD) / 2
• Monocular = (FPD – Right PD) / 2       and     (FPD – Left PD)  2

Where:

• FPD = Frame Pupillary Distance (A + DBL)
• PD = Patient’s Pupillary Distance

Example:

FPD = 72 mm
Binocular PD = 64 mm

(72 – 64) / 2   = 4 mm decentration per eye

Decentration must always have a direction. The key to remembering whether the horizontal decentration is In or Out is below:

• If the FPD is larger than the PD, then you will be decentering IN
• If the FPD is smaller than the PD, then you will be decentering OUT

Therefore, a frame with a PD of 72 mm and a patient with a binocular PD of 64 mm yields a decentration per eye of 4mm IN.

Vertical Decentration (Seg Drop) Calculation Formula:

• (B / 2)  – Seg Height or OC Height

Where:

• B = ‘B’ measurement of the frame
• OC Height = patient’s ocular center height
• Seg Height = patient’s segment height

Example:

B = 50 mm
Seg Height = 28 mm

(50 / 2)   – 28     = 3 mm (technically -3)

Again, when you have decentration you will have to give it a direction. With vertical decentration, you’re calculating Up or Down. The best way to remember the direction is to decide if the seg height is above (up) or below (down) the Datum Line. Remember, you are already calculating the Datum Line in the first portion of the formula: ‘B’ frame measurement divided by 2, aka half the ‘B’ measurement.

Therefore, a frame with a ‘b’ measurement of 50 mm and patient’s Seg Height of 28 mm yields a decentration of 3 mm  ABOVE/UP

• Horizontal & Vertical: The ABO/NCLE might test your understanding of both horizontal and vertical decentration. Remember vertical decentration is relevant for bifocals and progressive lenses.

• Induced Prism: Be prepared for questions about how decentration can create unwanted prismatic effects.

• Practice: Calculate decentration for various frame and PD combinations to solidify your understanding.

Practice Exercise- Find answers at the bottom

Determine the amount of horizontal decentration required for the following:

1.

“A” measurement = 54

DBL = 16

PD = 62

2.

“A” measurement = 42

DBL = 14

PD = 50

3.

“A” measurement = 50

DBL = 16

PD = 66

4.

“A” measurement = 48

DBL = 14

PD = 64

Calculate vertical decentration of the bifocal segment for the following:

5.

“B” Measurement = 52

Seg height = 23

6.

“B” Measurement = 48

Seg height = 24

7.

“B” Measurement = 50

Seg height = 28

8.

“B” Measurement = 46

Seg height = 19

REVEAL ANSWERS:

Vertical Decentration

Answers to Practice Exercises

1. 4 mm IN in each eye
2. 3 mm IN in each eye
3. 0
4. 1 mm OUT in each eye
5. 3 mm BELOW/DOWN
6. on line
7. 3 mm ABOVE/UP
8. 4 mm BELOW/DOWN

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