Normally, eyeglasses are fitted with the optical center of the lens directly in front of the eye. If the lens is fit off-center, image displacement can occur due to induced prism. The higher the power or the further the lens is fit off-center, the higher is the induced prismatic effect.
Prentice’s Rule:
Prism Power = Lens Power x Decentration (in centimeters)
Induced Prism
To better understand why there is induced prism, the cross-section of a plus lens can be likened to two prisms base-to-base, as the lens is thicker in the middle and thinner at the edges. Likewise, a minus lens can be likened to two prisms apex-to-apex, thinner in the middle and thicker at the edges. This induced prism can actually be used to the advantage of the lab when prism is called for in a prescription. This is known as prism by decentration.
Decentration = Prism * 10 / Power
This gives you the decentration in mm.
If (power0)
Base In Prism, decentration = –
Base Out Prism, decentration = +
Base Up Prism, decentration = –
Base Down Prism, decentration = +
Example:
-5.00sph lens need with 2.5∆ base in
Decentration = 2.5 x 10 / -5 = 5mm
As this is base In prism and the power is minus(<0), so that you have to add 5mm to the mono PD to get the required prism(2.5∆).
Note:
Important thing to remember: Prentice’s Rule breaks down for ophthalmic lens powers of less than about ± 1.00diopter.
Optometry Notebook 