Optics of Contact lenses by Ankit Varshney. If you understand optics properly you can prescribe contact lenses scientifically. Comparison between spectacles and contact lenses.
Basic subjective refraction for optometry, ophthalmology, ophthalmic technicians. Talk developed and given in 2016 to back office technicians in a 9 location ophthalmology practice consulting client to improve refraction outcomes.
Troubleshooting bifocals and Market Availability in Nepal
Bifocals in Anisometropia
Prismatic Effect in Bifocal
Bifocal Prescription
Bifocals in High Astigmatism
Basic subjective refraction for optometry, ophthalmology, ophthalmic technicians. Talk developed and given in 2016 to back office technicians in a 9 location ophthalmology practice consulting client to improve refraction outcomes.
Troubleshooting bifocals and Market Availability in Nepal
Bifocals in Anisometropia
Prismatic Effect in Bifocal
Bifocal Prescription
Bifocals in High Astigmatism
A DISSERTATION ON
“COMPARISION OF RESIDUAL ASTIGMATISM
FOLLOWING CONJUNCTIVAL AUTOGRAFT AFTER PTERYGIUM EXCISION, SUTURE VERSUS FIBRIN GLUE”
by Optom. Ankit Varshney.
COMPATIBILITY OF PROGRESSIVE GLASSES IN RELATION TO AGE, REFRACTIVE ERROR AND OCCUPATION OF PATIENT:
Journal: Sabargam International Journal of Research in Multidiscipline
ISSN: 2456-4672 Volume I, Issue II, Jan 2017
Orthoptic evaluation : Complete orthoptic evaluation for optometric students. Basics of orthoptics are explained here. Strabismus is evaluated in many ways, few of them have been discussed here
Paralytic squint. Here 4th nerve and 6th nerve palsy have been explained briefly. Even few important point related to its investigation and management is explained.
Restrictive strabismus is form of squint which is rare in origin. Here various form of restrictive squint have been explained, its diagnosis and its management is briefly explained.
Myopia management by Optom Ankit Varshney: Various ways to treat, manage (non optical & optical) for Myopic patients. Evidence based practice world wide.
New Directions in Targeted Therapeutic Approaches for Older Adults With Mantl...i3 Health
i3 Health is pleased to make the speaker slides from this activity available for use as a non-accredited self-study or teaching resource.
This slide deck presented by Dr. Kami Maddocks, Professor-Clinical in the Division of Hematology and
Associate Division Director for Ambulatory Operations
The Ohio State University Comprehensive Cancer Center, will provide insight into new directions in targeted therapeutic approaches for older adults with mantle cell lymphoma.
STATEMENT OF NEED
Mantle cell lymphoma (MCL) is a rare, aggressive B-cell non-Hodgkin lymphoma (NHL) accounting for 5% to 7% of all lymphomas. Its prognosis ranges from indolent disease that does not require treatment for years to very aggressive disease, which is associated with poor survival (Silkenstedt et al, 2021). Typically, MCL is diagnosed at advanced stage and in older patients who cannot tolerate intensive therapy (NCCN, 2022). Although recent advances have slightly increased remission rates, recurrence and relapse remain very common, leading to a median overall survival between 3 and 6 years (LLS, 2021). Though there are several effective options, progress is still needed towards establishing an accepted frontline approach for MCL (Castellino et al, 2022). Treatment selection and management of MCL are complicated by the heterogeneity of prognosis, advanced age and comorbidities of patients, and lack of an established standard approach for treatment, making it vital that clinicians be familiar with the latest research and advances in this area. In this activity chaired by Michael Wang, MD, Professor in the Department of Lymphoma & Myeloma at MD Anderson Cancer Center, expert faculty will discuss prognostic factors informing treatment, the promising results of recent trials in new therapeutic approaches, and the implications of treatment resistance in therapeutic selection for MCL.
Target Audience
Hematology/oncology fellows, attending faculty, and other health care professionals involved in the treatment of patients with mantle cell lymphoma (MCL).
Learning Objectives
1.) Identify clinical and biological prognostic factors that can guide treatment decision making for older adults with MCL
2.) Evaluate emerging data on targeted therapeutic approaches for treatment-naive and relapsed/refractory MCL and their applicability to older adults
3.) Assess mechanisms of resistance to targeted therapies for MCL and their implications for treatment selection
Title: Sense of Taste
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the structure and function of taste buds.
Describe the relationship between the taste threshold and taste index of common substances.
Explain the chemical basis and signal transduction of taste perception for each type of primary taste sensation.
Recognize different abnormalities of taste perception and their causes.
Key Topics:
Significance of Taste Sensation:
Differentiation between pleasant and harmful food
Influence on behavior
Selection of food based on metabolic needs
Receptors of Taste:
Taste buds on the tongue
Influence of sense of smell, texture of food, and pain stimulation (e.g., by pepper)
Primary and Secondary Taste Sensations:
Primary taste sensations: Sweet, Sour, Salty, Bitter, Umami
Chemical basis and signal transduction mechanisms for each taste
Taste Threshold and Index:
Taste threshold values for Sweet (sucrose), Salty (NaCl), Sour (HCl), and Bitter (Quinine)
Taste index relationship: Inversely proportional to taste threshold
Taste Blindness:
Inability to taste certain substances, particularly thiourea compounds
Example: Phenylthiocarbamide
Structure and Function of Taste Buds:
Composition: Epithelial cells, Sustentacular/Supporting cells, Taste cells, Basal cells
Features: Taste pores, Taste hairs/microvilli, and Taste nerve fibers
Location of Taste Buds:
Found in papillae of the tongue (Fungiform, Circumvallate, Foliate)
Also present on the palate, tonsillar pillars, epiglottis, and proximal esophagus
Mechanism of Taste Stimulation:
Interaction of taste substances with receptors on microvilli
Signal transduction pathways for Umami, Sweet, Bitter, Sour, and Salty tastes
Taste Sensitivity and Adaptation:
Decrease in sensitivity with age
Rapid adaptation of taste sensation
Role of Saliva in Taste:
Dissolution of tastants to reach receptors
Washing away the stimulus
Taste Preferences and Aversions:
Mechanisms behind taste preference and aversion
Influence of receptors and neural pathways
Impact of Sensory Nerve Damage:
Degeneration of taste buds if the sensory nerve fiber is cut
Abnormalities of Taste Detection:
Conditions: Ageusia, Hypogeusia, Dysgeusia (parageusia)
Causes: Nerve damage, neurological disorders, infections, poor oral hygiene, adverse drug effects, deficiencies, aging, tobacco use, altered neurotransmitter levels
Neurotransmitters and Taste Threshold:
Effects of serotonin (5-HT) and norepinephrine (NE) on taste sensitivity
Supertasters:
25% of the population with heightened sensitivity to taste, especially bitterness
Increased number of fungiform papillae
New Drug Discovery and Development .....NEHA GUPTA
The "New Drug Discovery and Development" process involves the identification, design, testing, and manufacturing of novel pharmaceutical compounds with the aim of introducing new and improved treatments for various medical conditions. This comprehensive endeavor encompasses various stages, including target identification, preclinical studies, clinical trials, regulatory approval, and post-market surveillance. It involves multidisciplinary collaboration among scientists, researchers, clinicians, regulatory experts, and pharmaceutical companies to bring innovative therapies to market and address unmet medical needs.
These simplified slides by Dr. Sidra Arshad present an overview of the non-respiratory functions of the respiratory tract.
Learning objectives:
1. Enlist the non-respiratory functions of the respiratory tract
2. Briefly explain how these functions are carried out
3. Discuss the significance of dead space
4. Differentiate between minute ventilation and alveolar ventilation
5. Describe the cough and sneeze reflexes
Study Resources:
1. Chapter 39, Guyton and Hall Textbook of Medical Physiology, 14th edition
2. Chapter 34, Ganong’s Review of Medical Physiology, 26th edition
3. Chapter 17, Human Physiology by Lauralee Sherwood, 9th edition
4. Non-respiratory functions of the lungs https://academic.oup.com/bjaed/article/13/3/98/278874
Acute scrotum is a general term referring to an emergency condition affecting the contents or the wall of the scrotum.
There are a number of conditions that present acutely, predominantly with pain and/or swelling
A careful and detailed history and examination, and in some cases, investigations allow differentiation between these diagnoses. A prompt diagnosis is essential as the patient may require urgent surgical intervention
Testicular torsion refers to twisting of the spermatic cord, causing ischaemia of the testicle.
Testicular torsion results from inadequate fixation of the testis to the tunica vaginalis producing ischemia from reduced arterial inflow and venous outflow obstruction.
The prevalence of testicular torsion in adult patients hospitalized with acute scrotal pain is approximately 25 to 50 percent
Report Back from SGO 2024: What’s the Latest in Cervical Cancer?bkling
Are you curious about what’s new in cervical cancer research or unsure what the findings mean? Join Dr. Emily Ko, a gynecologic oncologist at Penn Medicine, to learn about the latest updates from the Society of Gynecologic Oncology (SGO) 2024 Annual Meeting on Women’s Cancer. Dr. Ko will discuss what the research presented at the conference means for you and answer your questions about the new developments.
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Title: Sense of Smell
Presenter: Dr. Faiza, Assistant Professor of Physiology
Qualifications:
MBBS (Best Graduate, AIMC Lahore)
FCPS Physiology
ICMT, CHPE, DHPE (STMU)
MPH (GC University, Faisalabad)
MBA (Virtual University of Pakistan)
Learning Objectives:
Describe the primary categories of smells and the concept of odor blindness.
Explain the structure and location of the olfactory membrane and mucosa, including the types and roles of cells involved in olfaction.
Describe the pathway and mechanisms of olfactory signal transmission from the olfactory receptors to the brain.
Illustrate the biochemical cascade triggered by odorant binding to olfactory receptors, including the role of G-proteins and second messengers in generating an action potential.
Identify different types of olfactory disorders such as anosmia, hyposmia, hyperosmia, and dysosmia, including their potential causes.
Key Topics:
Olfactory Genes:
3% of the human genome accounts for olfactory genes.
400 genes for odorant receptors.
Olfactory Membrane:
Located in the superior part of the nasal cavity.
Medially: Folds downward along the superior septum.
Laterally: Folds over the superior turbinate and upper surface of the middle turbinate.
Total surface area: 5-10 square centimeters.
Olfactory Mucosa:
Olfactory Cells: Bipolar nerve cells derived from the CNS (100 million), with 4-25 olfactory cilia per cell.
Sustentacular Cells: Produce mucus and maintain ionic and molecular environment.
Basal Cells: Replace worn-out olfactory cells with an average lifespan of 1-2 months.
Bowman’s Gland: Secretes mucus.
Stimulation of Olfactory Cells:
Odorant dissolves in mucus and attaches to receptors on olfactory cilia.
Involves a cascade effect through G-proteins and second messengers, leading to depolarization and action potential generation in the olfactory nerve.
Quality of a Good Odorant:
Small (3-20 Carbon atoms), volatile, water-soluble, and lipid-soluble.
Facilitated by odorant-binding proteins in mucus.
Membrane Potential and Action Potential:
Resting membrane potential: -55mV.
Action potential frequency in the olfactory nerve increases with odorant strength.
Adaptation Towards the Sense of Smell:
Rapid adaptation within the first second, with further slow adaptation.
Psychological adaptation greater than receptor adaptation, involving feedback inhibition from the central nervous system.
Primary Sensations of Smell:
Camphoraceous, Musky, Floral, Pepperminty, Ethereal, Pungent, Putrid.
Odor Detection Threshold:
Examples: Hydrogen sulfide (0.0005 ppm), Methyl-mercaptan (0.002 ppm).
Some toxic substances are odorless at lethal concentrations.
Characteristics of Smell:
Odor blindness for single substances due to lack of appropriate receptor protein.
Behavioral and emotional influences of smell.
Transmission of Olfactory Signals:
From olfactory cells to glomeruli in the olfactory bulb, involving lateral inhibition.
Primitive, less old, and new olfactory systems with different path
1. 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?
7. 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.
14 July 2020 Optom. Ankit Varshney 7
9. 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
14 July 2020 Optom. Ankit Varshney 9
10. 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
14 July 2020 Optom. Ankit Varshney 10
11. 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
14 July 2020 Optom. Ankit Varshney 11
12. 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’
14 July 2020 Optom. Ankit Varshney 12
13. 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?
14 July 2020 Optom. Ankit Varshney 13
14. 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)
14 July 2020 Optom. Ankit Varshney 14
15. 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
16. 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
14 July 2020 Optom. Ankit Varshney 16
17. 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)
14 July 2020 Optom. Ankit Varshney 17
19. 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
14 July 2020 Optom. Ankit Varshney 19
21. TEAR LENS: FITTING RELATIONSHIPS - GENERAL
14 July 2020 Optom. Ankit Varshney 21
22. 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
14 July 2020 Optom. Ankit Varshney 22
23. 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
14 July 2020 Optom. Ankit Varshney 23
24. 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
14 July 2020 Optom. Ankit Varshney 24
26. CONTACT LENS OVER-REFRACTION
SOFT LENS
Ocular Rx = BVP + Over-Rx
Assumptions:
Lens conformance
Thin tear film under lens has zero
power
14 July 2020 Optom. Ankit Varshney 26
27. • 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
14 July 2020 Optom. Ankit Varshney 27
28. 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…
14 July 2020 Optom. Ankit Varshney 28
29. 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
14 July 2020 Optom. Ankit Varshney 29
30. 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
14 July 2020 Optom. Ankit Varshney 30
31. 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
14 July 2020 Optom. Ankit Varshney 31
32. 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
14 July 2020 Optom. Ankit Varshney 32
33. 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?
14 July 2020 Optom. Ankit Varshney 33
34. 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?
14 July 2020 Optom. Ankit Varshney 34
37. 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
14 July 2020 Optom. Ankit Varshney 37
38. 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}
14 July 2020 Optom. Ankit Varshney 38
40. 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?
14 July 2020 Optom. Ankit Varshney 40
41. 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
42. –An exophoric myope is at disadvantage with cls b/c there is no
prism
14 July 2020 Optom. Ankit Varshney 42
43. 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
44. 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
14 July 2020 Optom. Ankit Varshney 44
45. –An esophoric hyperope is at a disadvantage with cls b/c
there is no prismatic effect to counterbalance
14 July 2020 Optom. Ankit Varshney 45
46. 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
14 July 2020 Optom. Ankit Varshney 46
47. 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?
14 July 2020 Optom. Ankit Varshney 47
48. 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
50. 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
14 July 2020 Optom. Ankit Varshney 50
51. 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
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52. %SM or %CLM = (SM or CL- 1)*100
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53. %SM or %CLM = (SM or CL- 1)*100
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54. 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)
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55. 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
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56. RSM in Axial ametropia
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57. • 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.
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59. 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)
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60. 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
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61. 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?
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62. 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?
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63. 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?
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64. 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?
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65. 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
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66. 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
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67. 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
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68. 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%.
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Editor's Notes
Three basic CL fitting philosophies/relationships are possible & are shown here. The important issue optically is the tear lens formed between CL & cornea – STEEPER results in a PLUS tear lens, FLATTER in a MINUS tear lens, & when aligned, no significant tear lens. Some texts & practitioners use the term liquid lens in the place of tear lens – there is no difference other than the name. The choice is yours.
The last two calculations allow a useful & acceptably accurate Rule-of-Thumb to be created – basically a ∆ of 0.05 mm in radius (BOZR) produces a ∆ of 0.25 D in tear lens power. First principles (think of the physical model of what is happening) should be used to decide if the tear lens created has + or – power. Additional optical power of opposite sign is required in the CL to off-set the effects of the induced tear lens.
Starting with this slide several magnifications are introduced. Note that distant objects are assumed (zero-vergence incident light). Spectacle mag (SM) is particularly troublesome because a blurred, ametropic image is involved.
Regardless of the type of correction, image heights are proportional to the back vertex focal length of the correction. This is a diagrammatic representation of both spectacle & CL corrections in both myopia & hyperopia.
CLM can be calculated from a knowledge of the spectacle Rx & its vertex distance.
Similar calculations for myopes & hyperopes allow direct comparisons to be made of image sizes once the ametropia is corrected fully by CLs.