Decentration of lenses can induce unwanted prism. The amount of induced prism depends on the distance of decentration from the optical center and the power of the lens. For plus lenses, the base of the induced prism is in the direction of decentration, while for minus lenses it is in the opposite direction. Prism power can be calculated using Prentice's rule. The induced prism from decentration can have effects on binocular vision and eye alignment. Careful centration of lenses is important for optimal vision and comfort.

1. DECENTRATION AND ITS
EFFECT IN LENSES
By Prasamsha Dhungana

2. Introduction
Spherical lenses are considered to be
made of infinite number of prisms.
 Plus lenses have base to base and
 Minus lens have apex to apex
arrangement of prism.
Prismatic effect of the lenses increases with
increasing lens power and also distance
from the optical centre.

3. Optical centre
The optical centre(OC) of the lens is the
point at which light rays can pass with no
deviation.
When light goes through any other point
on a lens, the ray of light is bent.
It is important that a lens is ground so that
its optical centre is directly infront of the
patient’s pupil to allow optimum vision
through the lens.
There is least chromatic aberration and
distortion in optical centre.

4. A Decentered Lens
If the optical centre of the lens is
positioned over the pupil, the lens is
centered(there will be no prismatic
effect).
But if optical centre does not coincide
with the line of sight of the eye, then it
is called decentered lens and it
induces prism. It may be good or bad
depending upon the need of patient.

5. Induced prism
Prism can be created intentionally or
unintentionally.
If prism is prescribed for a patient,(like in case of
strabismus, convergence problems,
hemianopia) , then we induce it by decentering
intentionally.
But, if prism is not prescribed, errors in lab can
create unwanted prism.
With decentration, both prism power and prism
base are manifested.
The power of the prism depends on the amount of
lens decentration and the refractive power of
the lens being decentered.
The prism base orientation depends on the
direction of decentration and whether the lens is
positive or negative.

6. Prentice’s rule
It states that” the prismatic effect at any
point on a spherical lens is equal to the
distance of the point from the pole of the
lens, in centimeters, multiplied by the
power of the lens.”
P=c*F
where,
p= prismatic power of dioptres
c= amount of decentration in cm
F= power of lens

7. Major reference point
The point that has the desired amount
of prismatic effect in a lens, when
prism is prescribed.
For non-prism prescription, the MRP
and the optical centre of the lens are
same.

8. Monocular eye movement in
response to prism
Image of object is displaced towards the
apex.
Eyes moves towards the apex through
an angle equal to the angle of
deviation of the prism.
Eg. BO: eyes moves inward
BI: eyes moves outward
BU: eyes moves downward and
BD: eyes moves upward

9. Binocular eye movements in
response to prism
 When bases of prisms are in the same
direction
both eyes moves in the same
direction(versions)
Eg. Base in OD and
Base out OS
eye will turn to right.

10. When bases of prisms are in opposite
direction.
Eyes move in opposite direction
(vergences)
Eg.
Base in OU: eyes moves outwards
Base out OU: eyes moves inwards
Divergence due to base in prism
in both eyes.

11. Resultant horizontal prismatic
effects
When prisms make eye move in same
direction,(base in different direction):
The net effect is subtractive.
Eg. Base in in one eye and base out in
other eye.
OD= 3∆ BO and
OS= 5∆ BI
(move eyes in same direction, left side)
Net prismatic effect is 2∆BI.

12. When prisms make eye move in
opposite direction,(base in same
direction):
The net effect is additive.
Eg. Base out in both eyes.
OD=3∆ BO
OS= 4∆ BO
(moves eye in opposite direction)
Resultant prismatic effect will be 7∆ BO.

13. Resultant vertical prismatic
effects
When bases of prisms are in same
direction(both prisms with base up or
base down)
Net effect is subtractive.
Eg. OD= 4∆ BU
OS= 2∆ BU
Resultant prismatic effect is 2∆ BU.

14. When bases of prism are in opposite
directions(one base up and other base
down).
Net effect is additive.
Eg. OD= 2∆ BD
OS= 2∆ BU
Resultant prismatic effect is 4∆.

15.  Prism incorporated by decentration can
be either advantegeous or problematic,
depending on the situation.
 For plus lenses, base of induced prism
is towards the direction of decentration.
 For minus lens, base of the induced
prism is towards the opposite directon
of decentration.

16. Base of the induced prism in plus
lenses is towards the direction of
decentration.
A. If plus lenses decentered nasally:
Both eyes experience base in
prism.
IP
D
OCD

17. B. Plus lenses decentered temporally:
Both eyes experience
base out prisms.
IPD
OCD

18.  Minus lens decentered upward in
direction: IPD
OCD
Both eyes have base
down prism.
Base of the induced prism in minus
lens is opposite to the direction of
decentration.

19. When minus lenses decentered
downwards:
IPD
OCD
Here, both eyes will have
base up prism.

20. Few examples to find resultant
prismatic power and its base
direction
1.
If power of both eyes = +3.00DS and
Line of sight for eye passes 5mm nasal
to the optical centre of lens.
Prismatic effect=?
P=c*F
=0.5*3
= 1.5 ∆BO
5mm

21. 2.
Prescription
OD= -3.00DS
Distance PD=64mm and near PD= 60mm
Prismatic effect on each eye while
reading= ??
PD difference at distance and near= 64-
60mmi.e; 4mm
Pd difference for each eye=4/2= 2mm(it is
the amount of decentration as each eye
moves 2mm inward for reading)

22. Prismatic power in OD=
P=c*F
=0.2*3
=0.6 ∆BI
Prismatic effect in OS=
P=c*F
=0.2*3.5
=0.7∆BI
Net prismatic effect= 0.6+0.7=1.3∆ BI

23. Compounding prism power
When two prisms are combined in power
and base orientation to form one prism
that is the equivalent of both, the process
is known as compounding prism.
If two prisms are prescribed with prisms
base on horizontal and vertical direction,
we can compounded them into single
oblique one.
The resultant prism would be placed with
its base between the base direction of
two presribed prisms.
We can calculate it by using power of
prisms.

24. Examples on prism
compounding
Presriptiion:
OD= 3∆ BU and 4∆ BI
OS= plano
In the given figure, OV
represents vertical prisms,
OH represents horizontal
Prisms and OR represents
resultant prisms.
O
V
H
R

25. The exact position of the resultant prism can be
determined by using pythagoras prisms.
Calculation for left eye:
(OR)²= (OV)² + (OH)²
(OR)² = 3² + 4²
(OR)² =9+16
(OR)² =25
OR= √25
So, OR(power of resultant prism)= 5∆
Direction of base:
Tan(ROH)= ¾
ROH= tan-1 ¾
=36.87º ~37º
The resultant prism is: 5∆ base @ 37º

26. Resolving prism power
The process of expressing a single
oblique prism as two perpendicular
componenets is known as resolving
prisms.
If prisms are precribed in oblique
direction(like at 40˚),we can resolve it
into horizontal and vertical prism.
Example:
Prescription: OD=4∆ Base @30˚
OS= plano

27. 4∆
30˚
V
HO
Using simple trigonometry, we can resolve into
two different component.
For right eye,
Sin30= RH/OR
sin30= RH/4
RH= 4*sin30
=2∆ BU
Cos30= OH/OR
Cos30= OH/4
OH=4*cos30
=3.46∆ BI ~ 3.5∆ BI
So, the resultant prismatic power is:
R=2∆ BU and 3.5∆ BI
L= plano
30º
R
V=PsinØ
H=PcosØ

28. Decentration in spherocylindrical
lenses
Prismatic effect in cylindrical lenses is
experienced only if decentration occur in
power meridian. There would be no any
effect on axis meridian as there is no
power.
If axis of the cylindrical lenses are in
principal meridian i.e;90 or 180, prismatic
effect due to decentration can be
calculated same like in spherical lenses,
using Prentice rule.
But, if the axis of cylindrical power is
oblique, the calculation becomes more

29. Examples
Find the prismatic effect at a point 10 mm
below and 2mm nasal of the optical centre
of a +2.00/-1.00*90 lens (inRE)?
Horizontal prism:
P=c*F
=0.2*1
=0.2∆ BI
Vertical prism:
P=c*F
= 1*2
=2∆ BD
+2.00(V)
+1.00(H)

30. THANK YOU