How does decentration create prism?

When the optical center of a lens is displaced from the patient's line of sight, the patient looks through a point where prismatic effect exists (as described by Prentice's Rule). By intentionally shifting the optical center in a specific direction, the lab can produce the prescribed prismatic effect without grinding a separate prism into the lens. The amount of prism equals the decentration in centimeters times the lens power in diopters.

How do you calculate the decentration needed for a specific prism?

Rearrange Prentice's Rule: decentration (cm) = prism (Δ) / lens power (D). Then convert to mm by multiplying by 10. For example, to produce 2Δ in a +4.00 D lens: c = 2/4 = 0.5 cm = 5 mm. The optical center must be shifted 5 mm in the appropriate direction.

What direction should the optical center be shifted?

For a plus lens, shift the optical center in the direction of the prism base. For a minus lens, shift the optical center in the direction opposite to the prism base (toward the apex). This is because plus lenses have their base toward the thick part (optical center), while minus lenses have their base toward the thin edge (away from the optical center).

Prism by Decentration: Inducing Prism by Shifting the Optical Center

The Concept

Prism by decentration is a technique where prescribed prism is achieved by intentionally shifting the optical center of the lens away from the patient's pupil position, rather than grinding a separate prism into the lens. This works because looking through any point other than the optical center of a powered lens produces a prismatic effect, as described by Prentice's Rule.

This technique is commonly used in optical laboratories as an alternative to surfacing prism directly. It is simpler and less expensive, though it has limitations for large prism amounts.

Calculating Required Decentration

Starting from Prentice's Rule (Δ = c × D), rearranging gives:

c = Δ / D

where c is the decentration in centimeters, Δ is the prescribed prism, and D is the lens power.

Example 1: Prescribe 3Δ BI for a +6.00 D lens.

c = 3 / 6 = 0.5 cm = 5 mm decentration

Example 2: Prescribe 1.5Δ BU for a -5.00 D lens.

c = 1.5 / 5 = 0.3 cm = 3 mm decentration

For low-power lenses, large decentrations are needed to produce even small amounts of prism. For a +1.00 D lens, producing 2Δ requires 20 mm of decentration, which is impractical. Prism by decentration works best with moderate to high-power lenses.

Direction of Decentration

The direction to shift the optical center depends on the lens type:

Lens Type	Rule	Example
Plus lens	Shift OC toward the base direction	For BI prism, move OC nasally
Minus lens	Shift OC toward the apex direction (opposite the base)	For BI prism, move OC temporally

The logic: in a plus lens, the thick part (base) is at the center. Moving the center nasally places more "base" material nasally, creating base-in prism. In a minus lens, the thick part is at the edge. Moving the center temporally places the patient's line of sight closer to the nasal edge (the thick part), creating base-in prism.

Practical Considerations

When Decentration Works Well

Small prism amounts (1-3Δ) with moderate to high lens powers
When the required decentration is within the available lens blank diameter
For single-vision lenses (not progressive or bifocal)

When Decentration Is Impractical

Low-power lenses requiring large decentration (may exceed blank size)
Large prism amounts that require more decentration than physically possible
Progressive lenses, where decentration would displace the corridor and reading zone

When a lab cannot achieve the prescribed prism by decentration alone (often above 4-5Δ), they may use a combination of decentration plus ground prism. The lab should communicate this on the work order. Always verify the total prism on the finished lens with a lensometer.

Effect on Minimum Blank Size

Decentering the optical center shifts the usable portion of the lens blank relative to the frame. This means a larger blank diameter may be needed to ensure the lens covers the entire frame without cut-through (where the blank edge is visible within the frame). Labs calculate the minimum blank size (MBS) considering both the frame dimensions and the decentration.

Forgetting that prism by decentration changes the minimum blank size. If you order a lens with significant decentration without selecting a large enough blank, the lab may not be able to cut the lens to fit the frame. Always check with the lab when large decentrations are needed.

Key Takeaways

Prism by decentration shifts the OC to produce prescribed prism using Prentice's Rule.
Decentration needed: c (cm) = Δ / D, then convert to mm.
Plus lenses: shift OC toward the base. Minus lenses: shift OC toward the apex.
Works best for small prism amounts with moderate to high lens powers.
Large decentrations may require bigger lens blanks.
Not suitable for progressives; labs use ground-in prism for these.

Prism by Decentration: Inducing Prism by Shifting the Optical Center | ABO Exam