P1254325747uwWBJ

1
(No Transcript)
2
Page 2.11
Introduction to Ametropia
3
UABSO Class of 2011Demographics of Ametropia

1. Hyperopia (Far-sighted)
2. Emmetropic (no error/no distance correction)
3. Low Myopia (lt ?5 D)
4. High Myopia (gt ?5 D)

4
Demographics of Ametropia Class of 2011
5
Todays Goals

• Why do myopes need negative corrections?
• Why do hyperopes need positive corrections?
• Can we prescribe the same power in contacts and
  spectacles for
• a (given) low myope?
• a (given) high myope?

6
Introduction to Ametropia

• Are you near-sighted or far-sighted?
• Near-sighted (myopic)
• high or low?
• high
• How do you know how myopic you are?
• Whats the difference between a 2 D myope and an
  8 D myope?
• Before we can quantify ametropia, we have to set
  a standard for emmetropia

7
Standard Emmetropic Reduced Eye
Page 2.11
OBJ
8
Q1. A reduced eye with axial length 22.22 mm
requires 60 D reduced surface power to be
emmetropic. For an eye with 21.86 mm axial
length, the reduced surface power for emmetropia
would be

1. Greater than 60 D
2. 60 D
3. Less than 60 D

9
Emmetropia and Ametropia
Page 2.11
OBJ

• A longer eye needs lower power to be emmetropic

10
Emmetropia and Ametropia
Page 2.11

• A longer eye needs lower power to be emmetropic

11
Defining Ametropia
Page 2.12
Define ametropia in terms of the lens power that
will correct it
OBJ
12
Q2. We could define a myopic eye as one that is

1. Too long (only) for its power
2. Too strong (only) for its ax?
3. Both too long and too strong
4. None of the above

13
Defining Myopia

• A myopic eye has too much power (Fe exceeds
  power needed for emmetropia)
• OR, could say the eye is TOO LONG for its
  (reduced surface) power
• Myopia eye too strong or too long

14
Correcting Ametropia - the Far Point
(A) In myopia, light from a distant object
focuses in front of the retina
Page 2.13
OBJ
(B) In myopia, light from the Far Point focuses
on the retina
OBJ
Figure 2.8
15
Correcting Ametropia - the Far Point
Page 2.13

• The uncorrected myope readily identifies with the
  Far Point
• It is the furthest distance of clear vision
  (uncorrected)
• Objects beyond the Far Point appear blurred
• Objects at a range of distances inside the Far
  Point can be focused by accommodation

MR
?MR
Figure 2.8 (B)
16
In myopia, light from a distant object focuses in
front of the retina.In myopia, light from the
Far Point focuses on the retina.Q3. Therefore
spectacles would correct a myope for distance
vision by

1. Focusing objects that are at the Far Point on the
   retina
2. Diverging parallel incident light so that the
   center of curvature of the divergent waves is at
   the Far Point
3. Converging parallel incident light so that it
   focuses in front of the retina
4. Diverging parallel incident light so that the
   center of curvature of the divergent waves is at
   the retina

17
Quantifying Ametropia
Page 2.15
e.g. Far Point (MR) 50 cm in front of the eye
MR
?MR
Far Point vergence is equal and opposite to the
myopic eyes power excess We correct an ametropic
eye with a lens equal and opposite to its power
excess ? LMR A (Ametropia). Taking an eye with
the standard 22.22 mm axial length
The eye has standard axial length, so we could
define this as ?2.00 D refractive ametropia
(indicating that it differs from the SERE in
refractive power only)
18
Quantifying Ametropia (cont.)
Page 2.15
Another eye with ?2.00 D myopia Fe 61 D
This eye has ?1.00 D refractive myopia and ?1.00
D axial myopia
19
Spectacle Correction and the Far Point

• Provided light reaches the eye with Far Point
  vergence, a clear retinal image results
  (unaccommodated eye)
• It does not matter how Far Point vergence is
  produced
• by a real object at the Far Point
• by a spectacle lens that diverges light from a
  distant object so that incident vergence at the
  eye equals Far Point vergence
• by a contact lens that produces Far Point
  vergence fromlight

OBJ
Page 2.13
MR
?MR
Figure 2.8 (B)
20
Equivalence of Far Point Vergence Ametropia
Page 2.16
Optically, the eye sees no difference between a
real object in the Far Point Plane and incident
light diverged by a spectacle lens to produce Far
Point vergence at the eye (reduced surface)
Negative spectacle lens producing far point
vergence at the plane of the eye (reduced surface)
Light incident at the eye with far point vergence
focuses at the retina (unaccommodated)
Figure 2.9
21
Q5. Which correction would have the higher
power for a given myopic patient?

1. Spectacles
2. Contact lenses

22
Equivalence of Far Point Vergence Ametropia
Page 2.17
Light waves demonstrate the equivalence
between(A) divergence of light to produce Far
Point vergence at the eye, and (B) divergence at
the eye from a real object at the Far Point
Figure 2.10
23
Spectacle vs. Contact Lens vs. Ocular Correction
Page 2.17
Figure 2.10
24
Spectacle vs. Contact Lens vs. Ocular Correction
Page 2.17
FS
FCL
FO
Figure 2.10
25
Spectacle vs. Ocular Correction Examples
Page 2.17
FS
d
vertex distance
Figure 2.10
26
Spectacle vs. Ocular Correction
Page 2.18
LOW MYOPIA EXAMPLE
??S
?MR
27
Spectacle vs. Ocular Correction
Page 2.18
HIGH MYOPIA EXAMPLE
??S
?MR
28
Hyperopia and the Far Point
Page 2.19
Figure 2.11 The uncorrected hyperopic eye has
too little power, so parallel incident light
focuses behind the retina (or would focus there
if it were not for the presence of the retina).
Convergent incident light is therefore needed to
move the image forward to the retina.
29
In hyperopia, light from a distant object focuses
behind the retina.Q5. Spectacles would correct
a hyperope for distance vision by

1. Diverging parallel incident light so that is
   converged by the eye to focus in front of the
   retina
2. Diverging parallel incident light. Light is then
   converged by the eye to focus at the retina
3. Converging parallel incident light. Light is
   then further converged by the eye to focus at the
   retina
4. Converging parallel incident light to overcome
   the net divergent power of the eye to focus at
   the retina

30
Page 2.20
Far Point Vergence in Hyperopia
Underpowered hyperopic eye requires convergent
incident light at the reduced surface to focus
the image on the retina (unaccommodated)
Convergent incident light (in air) is traveling
toward a virtual Far Point (object) Plane
Figure 2.12
31
Spectacle Correction in Hyperopia
Page 2.21
Positive spectacle lens power converges
incident light toward the Far Point Plane (in air)
Figure 2.13
32
Spectacle vs. Ocular Correction Examples
Page 2.21
?MR
d
33
Spectacle vs. Ocular Correction
Page 2.21
?MR
d
34
Far Point, Eye Movements Spectacle Lenses
Page 2.23
As the myopic eye rotates, the Far Point traces
out a spherical surface, the Far Point Sphere
Figure 2.14
35
Far Point, Eye Movements Spectacle Lenses
Page 2.24
In the spectacle-corrected patient, we want the
image produced by the spectacle lens (from
incident light) to fall on the Far Point Sphere
for all directions of gaze This is one of the
tenets of corrected curve ophthalmiclens
design
Figure 2.15
36
Recap Key Objectives - Ametropia

• Ametropia is a mismatching of ocular power and
  axial length
• The Far Point of any (uncorrected) eye is
  conjugate to the retina for distance vision
• Ametropia is corrected by placing the second
  focus of the correcting lens at the Far Point

37
Todays Goals

• Why do myopes need negative corrections?
• Why do hyperopes need positive corrections?
• Can we prescribe the same power in contacts and
  spectacles for
• a (given) low myope?
• a (given) high myope?

Eye has too much power, or is too long (distant
object focuses i.f.o. retina)
Eye has too little power, or is too short
(distance object focuses behind retina)
Probably (powers almost the same)
No (difference exceeds 0.25 D)