Optics of contact lenses by ankit varshney

Optom. Ankit S. Varshney
M.Optom, Ph.D. in Optometry (pursuing) Fellow of IACLE (Aus.), Fellow of ASCO(Mum.)
Prof. at (Shree Bharatimaiya College of Optometry & Physiotherapy, Surat)
Educator Member of International Association of Contact lenses Educators (Australia)(IACLE)
Associate Member of Association of Schools and Colleges of Optometry(ASCO)
Life Member of Indian Optometric Association (IOA)
Member of Optometry Council of India(OCI)
Mail id: ankitsvarshney@yahoo.com
Whatsapp no.: +918155955820
Do we see better with CL’s
compared to glasses?

EVIDENCEBASEDPRACTICE:
MEETING THE EXPECTATIONS

While deciding the treatment
for ametropia..

Expectations..
Contact lenses is..
14 July 2020 Optom. Ankit Varshney 4

An Optometrist Should
Make the patient feel comfortable14 July 2020 Optom. Ankit Varshney 5

An Optometrist Should
Know optics of contact lenses14 July 2020 Optom. Ankit Varshney 6

1. Are CLs are thin lenses?–
No, CLs are thick lenses
Although thin in appearance , CL are treated in
geometrical optics as a “thick lens”.
Unlike thin lenses , the refraction of light as it
passes through the thickness of the lens must be
taken into consideration.
Thin Lenses:
Fthin= F1 + F2
Thick Lenses (Equivalent Lens Power):
Fthick = F1 + F2 – t/n (F1*F2)
* CLs surfaces are highly curved, thus sagitta of lens considered to
be large in terms of chord length.

LIGHT
n n¢
rsurf
Optic axis Centre of curvature

SURFACE POWERS
For example:
• n = air = 1.00
• nl RGP = 1.44
• rSurf = +7.80 mm(0.0078 m)
• Fsurf = +56.41 D
Fsurface = (nl -n)
rsurface

f
F
H H’
fFVP
fT
A1 A2
t
F’
f’T
fBVP
n’n
Extreme curves in CLs
1 - (F1)
BVP=In practice we measure the position of second
principal focus from the back vertex of the lens since this is accessible
.The power so measured is the Back Vertex Power .
BVP= + F2
F1
tc
nLens

2. Why is the power different from glasses to CLs–
Effectivity/Effective Power
 Changes from spectacle plane to corneal
plane.
 Always relatively more plus at corneal plane.
– Myopes need less (-)power in cls
– Hyperopes need more (+)power in cls
 Remember to consider vertex distance for
all powers > ± 4.00 D
 Also look for the cylinder component.
– Use optical crosses

THE EFFECTIVITY RELATIONSHIP= How to convert spectacle to Cls?
F
d f - d
f
LIGHT
Vergence @ F = , Vergence @ F  = & f=1/F ; F’=
1
f
1
f- d
F
(1 - dF)
F’

 In thick lenses:
 n=1 & Ftotal =0, thus Fcl= FSp /1-dFSp
1 - (F1)
Ftotal= + F2
F1
tc
nLens
1 - (Fsp)
Ftotal = + Fcl
FSp
d
n
THE EFFECTIVITY RELATIONSHIP= How to convert spectacle to Cls?

CORRECTION FOR VERTEX
DISTANCE
FC/L=
d = Distance from back vertex of spectacle lens
to corneal apex (10-14 mm generallyapply)
FSp
(1 - d FSp)

Calculations of V.D.
 FCL= FSP/1-dFSP use this to convert from specs to contacts
 FSP= FCL / 1+d FCL use this to convert from CLs to
spectacles14 July 2020 Optom. Ankit Varshney 15

3. Why is the power different from glasses to CLs–
Tear lens
Lenses can alter the shape of the cornea
(warpage)
Cornea can alter the shape of lenses (soft cls)
RGP lenses can mask corneal astigmatism
CONTACT LENSES ON A CORNEA

OPHTHALMOMETRY:
PURKINJE- SANSON IMAGE #1
Ophthalmometers (keratometers) measure RADIUS of
Curvature, NOT dioptric power
Range of corneal curvature: Approximately 7.1 -
8.7 mm
 Indicated dioptric power based on assumption ncornea = 1.3375
For a 7.8 mm cornea:
Dioptric power = 48.205 (n=1.376)
‘K reading’= 43.269 (n=1.3375)

CONTACT LENSES ON A CORNEA
Tear lens under a flexible lens is very
thin and has no power
Tear lens under a rigid lens depends on
material rigidity and the fitting
relationship
If a rigid lens decentres, the tear lens
will acquire a prismatic component

Lid
Rigid Contact
Lens
‘Prismatic’
Tear Lens
Cornea

TEAR LENS: FITTING RELATIONSHIPS - GENERAL

TEAR LENS POWER RIGID LENSES
TL/Air front interface power =
If flatter by 0.05, interface power =
1.336 - 1.000
0.0078
= +43.077 D
1.336 - 1.000
0.00785
= +42.803 D
Flatten BOZR by 0.05 mm
Similarly, Steeper by 0.05mm
D-0.274 D in TL power
D+0.278 D in TL power
Assume tear lens (TL) to be in air, BOZR = 7.80 mm,
nTear = 1.336.
RULE-OF-THUMB:
• ∆s in a CL’s BOZR of 0.05 mm  ∆s in the Tear Lens power
by approximately 0.25 D
• FLATTER  –0.25 D
• STEEPER  +0.25 D

When the BOZR of a rigid CL is FLATTENED by 0.05 mm:
 The Tear Lens power is altered (decreased) by –0.274 D
When the BOZR of a rigid CL is STEEPENED by 0.05 mm:
 The Tear Lens power is altered (increased) by +0.278 D
SAM & FAP Rule
BVP= -3.00 D & Initial BC = 7.80mm,
new BC= 7.75mm, Flatter by 0.05mm (-0.25D)TL
(FAP) rule
NEW BVP = -3.00 +0.25= -2.75D
BVP= +3.00 D & Initial BC = 7.80mm,
new BC= 7.75mm, Flatter by 0.05mm (-0.25D)TL
(FAP) rule

SAM & FAP Rule
BVP= -3.00 D & Initial BC = 7.80mm,
new BC= 7.85mm, Steeper by 0.05mm (+0.25D)TL
(SAM) rule
NEW BVP = -3.00 + -0.25= -3.25D
BVP= +3.00 D & Initial BC = 7.80mm,
new BC= 7.85mm, Steeper by 0.05mm (+0.25D)TL
(SAM) rule
NEW BVP = +3.00 + -0.25= +2.75D

CONTACT LENS OVER-REFRACTION
RIGID LENS
Ocular Rx =
BVPTrial + Tear Lens Power + Over-Rx
TL= -K + B.C. (-44.00 +43.00)= -1.00
TC = BVP + TL (-3.00+-1.00)= -4.00
OR = Rx – TC (-3.50 - -4.00)=+0.50
Final Rx = BVP + OR (-3.00 + +0.50)=-2.50
For verification= Final Rx (New BVP + TL)

CONTACT LENS OVER-REFRACTION
SOFT LENS
Ocular Rx = BVP + Over-Rx
Assumptions:
 Lens conformance
 Thin tear film under lens has zero
power

• Spherical GP neutralizes corneal cylinder
& Residual astigmatism =
(refractive cylinder - corneal cylinder)
Examples
 K‟s: 41.00/42.25@090 (+1.25X090)
Rx: -4.50+1.25X090
RA = (+1.25X090) – (+1.25X090) = (+0.00X090) Spherical GP or toric
SCL
 K‟s: 43.50/44.25@090 (+0.50X090)
Rx: -3.25+2.25X090
RA = (+2.25X090) – (+0.50X090) = (+1.75X090) Toric SCL or GP front
toric
 K‟s: 42.50/45.50@090 (+3.00X090)
Rx: -5.50+4.50X090
RA = (+4.50X090) – (+3.00X090) = (+1.50X090) Toric SCL or Bitoric GP

The corneal reflex with a contact lens is a superimposition of
reflections from the air/tear interface, the anterior and posterior
tear/lens interfaces and the tear/corneal reflex.
Fresnel’s formula of reflection
R={n’-n/n’+n}2
4. Why the corneal reflex is brighter in a person
with contacts (no ARC in CLs)?- An optical
interval…

REFRACTIVE INDICES OF CONTACT
LENS MATERIALS
PMMA CLs (trial lenses) 1.49
GP CLs 1.48 – 1.41
SCLs 1.44 – 1.38
SiHy CLs 1.40 – 1.426

PURKINJE-SANSON IMAGE #1
n = 1.000 (air)
n = 1.336 (tears)
R = 0.0207
Light loss approx. due to tears=
2.1%
R={n’-n/n’+n}2

Air/tear interface Tear/lens interface
CONTACT
LENS
CORNEA
AQUEOUS
AIR
Prelens tear film
Post-lens tear film
Tear/epithelial interface
For practical purposes
think of each layer as
a separate lens in air

REAL LIGHT LOSSES: C/L SYSTEMS
Sum of (S):
Air/Tears, Tears/CL,
CL/Tears, Tears/Cornea
LOSS= A/T + T/CL+ CL/T + T/C
RGP n = 1.48 S = 2.6%
RGP n = 1.41 S = 2.2%
SCL n = 1.44 S = 2.4%
SCL n = 1.38 S = 2.1%
• Air/tear film interface is main contributor to loss in all cases
R={n’-n/n’+n}2

 Hyperopes have more accommodative
demand with glasses
– Pre-presbyopes love cls! (more plus with CLS)
 Myopes have less accommodative demand
with glasses
– Pre-presbyopes do not do well (more minus to
overcome with CLS)
5. What will happen in accommodative &
convergence demand with CLs?

Ex. The pt. has a ±7.00 D spects correction at
12mm VD. When fitted with CL, will pt
requires more or less accommodation, for a
33 cm viewing distance than spects? What
dioptric amount of accommodation would
this pt requires with spects & with CL?

 If a myope is switched FROM spectacles TO contact
lenses, the change may PRECIPITATE the need for a near
correction
– in myopia, specs  CLs can  any existing problem
 If a hyperope is switched FROM spectacles TO contact
lenses, the change may POSTPONE the need for a near
correction
– in hyperopia, specs  CLs can  any existing problem
ACCOMMODATION: INCIPIENT PRESBYOPIA

CL optics –Changes in accommodative
convergence
 Myopes = increased accommodation with cls , thus
will have MORE accommodative convergence
– An esophoric myope will have to use more
NEGATIVE fusional vergence (divergence) {thus eso
pt. disadvantage}
 Hyperopes= decreased accom with cls, will have
LESS accommodative convergence
– An exophoric hyperope will have to use more
POSITIVE fusional vergence (convergence){thus exo
pt. disadvantage}

Prismatic effect is induced if the line of sight
does not pass through the optical centre of a
lens
The prismatic effect can be calculated by
Prentice’s Rule:
Prism (D) = (c xF) Lens Power x Decentration
(cm)
6. What changes will happen due to prismatic
effect in NEAR vision?

CL Optics- Prismatic Effects
Minus lenses induce BI prism
Less convergence needed
–An exophoric myope is at disadvantage with cls b/c there is
no prism
The lack of prism effect is a benefit for anisometropes.14 July 2020 Optom. Ankit Varshney 41

–An exophoric myope is at disadvantage with cls b/c there is no
prism

Myopic convergence
A myope wearing contact lenses converges more than when
wearing spectacles. When wearing spectacles, they behave as a
base-in relieving prism, and the eye converges less than the
viewing distance would suggest.
Apparent object
position
(spectacles) Contact lens-wearing
myope converges
MORE
Near object
Distance PD14 July 2020 Optom. Ankit Varshney 43

CL Optics- Prismatic effects
 Correctly fitted cls are always centered on the eye, where glasses
induce prism
 Plus lenses induce BO prism
–An esophoric hyperope is at a disadvantage with cls b/c
there is no prismatic effect to counterbalance

–An esophoric hyperope is at a disadvantage with cls b/c
there is no prismatic effect to counterbalance

HYPEROPIA – CONVERGENCE
A hyperope wearing contact lenses converges less than when
wearing spectacles. This is because of the base-out prism effect
induced by spectacles acting as an exercising prism which forces
more convergence than the viewing distance would suggest
Apparent object position
(spectacles)
Contact lens-wearing
hyperope converges LESS
Near object
Distance PD

Less Occurs because the cl is touching the
cornea
The power factor of SM formula includes the
distance from lens to entrance pupil, changing
this to zero causes a change in magnification
Myope will get larger retinal image
Hyperope will get smaller retinal image
The opposite of what happens in glasses!
7. What will happen to magnification in CLs?

SPECTACLES vs CONTACT LENSES:
MAGNIFICATIONS
Spectacle Magnification (SM) =
Corrected ametropic image size
Uncorrected ametropic image size
SM is a comparison of a clear retinal image with a blurred retinal image
Contact Lens Magnification (CLM) =
Corrected image size with CLs
Corrected image size with specs
CLM is a more realistic comparison of two clear retinal image sizes
Distant objects assumed14 July 2020 Optom. Ankit Varshney 48

SPECTACLES vs CONTACT LENSES

CONTACT LENS MAGNIFICATION (CLM)
C SReferring to right angled triangles CFh & SFh
Image size with CL
Image size with spectacles
But image sizes are directly proportional to the focal lengths
Therefore...
'
or
'
C
S
CL
Spec
CLM
Fh
CLM
Fh
f
CLM
f



'
'
However, we have already shown that:
'
'
(1 ' )
'
'
(1 ' )
Simplifying...
1 '
Spec
CL
Spec
CL
Spec
Spec
Spec
Spec
Spec
F
CLM
F
F
F
dF
F
CLM
F
dF
CLM dF





 

 From its derivation, CLM is a comparison of contact lens & spectacle lens
image sizes
 Calculating examples using a vertex distance of 14 mm (d = 14 mm):
 For a +10 D HYPEROPE, CLM = 0.86 (i.e. 14% SMALLER)
 For a –10 D MYOPE, CLM = 1.14 (i.e. 14% LARGER)
 These CLMs show that hyperopes experience a smaller image size with
contact lenses than with spectacles of equivalent
 Conversely, myopes experience a larger image size than with spectacles
1 SpecCLM dF 
CONTACT LENS MAGNIFICATION (CLM)
%SM or %CLM = (SM or CL- 1)*100

RELATIVE SPECTACLE MAGNIFICATION (RSM)
RSM:
For a distant object, the RSM is the ratio of image size in
the corrected ametropic eye to the image size in the
NORMAL emmetropic eye.
Note: fEye = –(g + d2)

RSM in Refractive ametropia
• If the source of the ametropia is assumed to be refractive
• RSM = 1 + d2FSP
• With Spectacles (d2 ≈ d = vertex distance): [RSM ≠ unity]
• With Contact Lenses, d2 = 1.55 mm. In this context 1.55 mm
is treated as being negligible (≈ 0). [RSM ≈ unity]
• Clearly, if anisometropia results from ametropia (unilateral
or bilateral) which is refractive in origin, contact lenses
would be the correction of choice because they produce
negligible differences between the corrected image size and
the normal emmetropic image size
2
Approximations are:
1 for REFRACTIVE ametropia
1 for AXIAL ametropia
Spec
Spec
RSM d F
RSM gF
 
 

RSM in Axial ametropia

• Knapp’s Law:
• For axially ametropic eye , if the correcting lens is placed
so that its secondary principal point coincides with
anterior focal point of the eye , the size of retinal image is
same as if it were standard emmetropic eye.
• Clearly, if anisometropia results from ametropia (unilateral
or bilateral) which is axial in origin, spectacles would be the
correction of choice.

 Spectacle magnification
•Axial myopia = spec > cl
•Axial hyperopia = spec > cl
•Refractive myopia = cl >spec
•Refractive hyperopia = cl > spec

 AXIAL ametropia: correct with SPECTACLES
 REFRACTIVE ametropia: correct with CLs
RSM: SUMMARY
Implication: CLs are not always the correction of
choice
Clinical judgement is required
Refractive surgery procedures, e.g. LASIK, are
REFRACTIVE in origin (cornea-based
corrections)

 Higher ametropias (4 to 8 D) are more likely to
be AXIAL
– spectacle correction may be better in cases of
axial anisometropia
 Importantly/fortunately, most ametropias
are approximately isometropic, not
anisometropic
– other considerations may apply, e.g. cosmetic,
occupational, & safety issues
AMETROPIAS: AXIAL OR REFRACTIVE

 Astigmatism is classed as a refractive ametropia
(one meridian can be considered ‘normal’, the
other meridian the ‘cause’ of the astigmatism)
 Spectacle lenses can induce significant
meridional aniseikonia, especially in high
astigmatism.
 CLs are the correction of choice in astigmatism
despite some fitting issues.
WHAT ABOUT ASTIGMATISM?

 APHAKIA is considered to be refractive in origin
 IOL implantation is considered to be the ideal
correction. Eye  pseudophakic (location of Rx
nearly ideal)
– IOLs impart optical & physiological advantages
– If IOLs are not implanted, CLs are preferable
WHAT ABOUT APHAKIA?

All are retinal image size comparisons
 SM compares corrected (focused) with uncorrected
(blurred) images – difficult!
 CLM compares images with CL & spectacle corrections -
more realistic
– RSM compares corrected with a theoretical, emmetropic,
schematic eye - hypothetical (axial, refractive, mixed, or
unknown?)
HOW DO SM, CLM, & RSM RELATE TO ONE ANOTHER?

 CL wearers have greater field of view
– No glasses rim!
 Spectacles suffer from oblique astigmatism,
curvature of field and distortion
– CLS eliminate OA and CF because the lenses
are always centered
– CLS eliminate D because they are directly on
the eye
8. What will happen to aberrations and field of view
in CLs?

ABC’s of lens aberrations
 Low order aberrations
•Sphere, cylinder
 Higher order aberrations
When considering off-axis rays
A=Astigmatism of Oblique Incidence
B=Barrel/pincushion distortion
C=Coma/Chromatic aberration
S=Spherical aberration

FIELD LIMITATIONS HYPEROPIA
Lens-limited rays
Ring SCOTOMA Ring SCOTOMA
Centre of
rotation
Centre of
entrance
pupil
Field of view Field of fixation
Appliance-limited
rays
CLCL
SLSL

FIELD LIMITATIONS MYOPIA
Appliance-limited rays
Ring DIPLOPIA Ring DIPLOPIA
Centre of
rotation
Centre of
entrance
pupil
Field of view Field of view
Lens-limited
rays
CLCL
SLSL

Summary
Advantages Disadvantages
No astigmatism of oblique pencils. Lens decentration produces ‘ghosting’
or flare from the peripheral zone of the
lens
No distortion When a toric lens rotates, a toric
over-refraction and decreased vision
may result.
No chromatic aberration Moving or generally unstable lenses
may produce disturbances of vision
No limitations on the field of view In axial ametropia, usually
spectacles are better suited
No spectacle frame diplopia. (Myopia).
No spectacle frame scotoma. (Hyperopia).
No prismatic imbalance in anisometropia
Corneal irregularities/astigmatism
reduced by 90%.

Optics of contact lenses by ankit varshney

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