Transcript Changes in retinal image size

AREA 2: Fitting single-vision lenses
• Topic 4: Alignment, decentration, and minimum
diameter
• Topic 5: Calculating thickness in spherical lenses
• Topic 6: Calculating thickness in astigmatic lenses
• Topic 7: How lenses affect vision
• Topic 8: Prismatic effects
• Topic 9: Base rules
• Topic 10: How alignment affects binocular vision
Topic 7: How lenses affect vision
• Changes in visual field
• Changes in retinal image size
• Effect of vertex distance
Changes in visual field
PE
Peripheral visual field
z’
Central visual field
Changes in visual field
S
S
60º
60º
60º
T
95º
95º
T
N N
60º
75º
75º
I
I
OD
OS
Changes in visual field
Reduction of visual field by frame
C
A


‘
z’
tg  ' 
l2'
Apparent visual field

2
L'
Changes in visual field
Reduction of visual field by lens vergence
CR
CA
CA
CR


‘
z’ 
z

l2'
 z ‘
l2
l 2'
l2
Plus lens
Minus lens
Real visual field
z’
Changes in visual field
Calculation of real visual field (2)
tg 

2
L' BVP 
Changes in visual field
Scotoma in plus lenses
Changes in retinal image size
Spectacle
magnification
Changes in retinal image size
Spectacle magnification:
Retinalimage size WIT HOUTlens tg
SM 

Retinalimage size WIT Hlens
tg '
H

H’
’
R = F’

’

x
f’-x
f’
h’
Changes in retinal image size
f´vp
E
dv
ec
F´= R
3 mm
d
Reverse afocal Galilean
viewfinder
Thin lens
Changes in retinal image size
Afocal Galilean
viewfinder
F 1
F2
f2
ec
f 1
Ms 
1
 
P1  
1   Tc   
n 
 
Changes in retinal image size
Thin lens
f vp
H H
E
dv
3 mm
d x
1
Mp 
1  d  BVP
F´= R
Changes in retinal image size
Spectacle magnification:




1

  1 
SM  

 1  d  BVP   1  Tc P 

1
n 

Mp
Ms
Changes in retinal image size
BVP (D)
Tc (mm)
P1 (D)
Ms
Mp
SM
% SM
-20.00
0.70
0.00
1.000
0.758
0.758
-24.2%
-15.00
0.70
0.00
1.000
0.806
0.806
-19.4%
-10.00
0.80
+3.00
1.002
0.862
0.864
-13.6%
-5.00
0.80
+4.49
1.002
0.926
0.928
-7.0%
0
1.80
+5.96
1.007
1.000
1.007
0.7%
+5.00
4.50
+10.19
1.031
1.087
1.121
12.1%
+10.00
7.00
+12.27
1.060
1.190
1.216
26.1%
+15.00
8.50
+13.84
1.084
1.316
1.426
42.6%
Changes in retinal image size
Spectacle
magnification
Changes in retinal image size
Anisometropia and aniseikonia
Changes in retinal image size
Anisometropia and
aniseikonia
Changes in retinal image size
Aniseikonia
• <3%
No symptoms
• 3-5%
Asthenopia
• >5%
Problems with binocular
vision
Changes in retinal image size
Aniseikonia caused by anisometropia
Tc
P1
P2
BVP
2.04
+5.21
-4.25
+1.00
n = 1.523
vd = 15 mm
Ø = 65 mm
OD
OS
5.28
+7.78
-4.00
+4.00
n = 1.8
vd = 12 mm
Ø = 65 mm
OD
1.66
+5.22
-4.25
+1.00
OS
3.68
+7.87
-4.00
+4.00
n = 1.8
vd = 12 mm
Ø = 55 mm
OD
1.9
+5.22
-4.25
+1.00
OS
1.9
+5.22
-1.25
+4.00
 SM
7.1%
5%
3.8%
Changes in retinal image size
Aniseikonia
greater than
5%
• Myopia, anisometropia > 3.00 D
• Hyperopia, anisometropia > 2.00 D
Changes in retinal image size
Distortion in astigmatic lenses:
+ 6.00
+ 4.00
+ 4.00
OBJECTE
90º+ 2.00+ 4.00
+ 6.00
Changes in retinal image size
SMmax  SMmin
FAA  100
SMmax
SMmax

SMmin
Changes in retinal image size
Angular distortion
+6.00
120º-2.00+6.00 D
+4.00
OBJECTE
Changes in retinal image size
Effect of angular distortion
Cylinder axis expressed as plus cylinder
Image
Object
Changes in retinal image size
+4.00 D
+6.00 D
Object
50º +2.00 +4.00 D lens
+4.00 D
+6.00 D
Effect of vertex distance
R= F’
e
f L2
f L1
fL1  fL2  e
1
1
PL 2
PL1 


1
fL2  e
 e 1  e  PL 2
PL 2
Effect of vertex distance
Example: New frame
PLo 2
PLo 1

1  e  PLo 1
e  d 2  d1
Myope: Glasses prescription with vd of 12 mm, -9.00 D
PL15 
PL12
1  3  103  PL12
Glasses prescription with vd of 15 mm, -9.25 D
Effect of vertex distance
Example: Fitting a contact lens (above 5.00 D)
R= F’
e
f LC
dv
f Lo 1
PLC 
f Lo P
2
Lo
1  dv ·PLo
Effect of vertex distance
Example: Signs
Movement of glasses
to see better
Hyperopia
Myopia
Closer to the eyes
Overcorrected
Undercorrected
Away from the eyes
Undercorrected
Overcorrected
Effect of vertex distance
Example:
Signs