area 2: fitting single-vision lenses

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 fieldChanges in visual field

• Changes in retinal image sizeChanges in retinal image size

• Effect of vertex distanceEffect of vertex distance

Peripheral visual field Central visual field

PE z’

Changes in visual field

60º

75º

95ºT N

S

I

60º

95º

75º

T

S

I

OD OS

Changes in visual field

60ºN

60º

Reduction of visual field by frame

z’ ‘

'2l

C A

Changes in visual field

Apparent visual field

'' Ltg2

Reduction of visual field by lens vergence

Plus lens Minus lens

z’ z‘

'2l

2l

C R

C A

z’z ‘

'2l

2l

C R

C A

Changes in visual field

Real visual field

BVPLtg '2

Calculation of real visual field (2)

Changes in visual field

Changes in visual field

Scotoma in plus lenses

Spectacle magnification

Changes in retinal image size

Spectacle magnification:

’

’

H H ’

f ’- x

f ’ x

h ’

R = F’

'lens WITHsize image Retinal

lens WITHOUTsize image Retinal

tg

tgSM

Changes in retinal image size

F´ = RE

f´ v p

d v

de c

3 m m

Reverse afocal Galilean viewfinder

Thin lens

Changes in retinal image size

1F

2F

ce 2f

1f

nP

T

Ms

c11

1

Afocal Galilean viewfinder

Changes in retinal image size

F´ = RE

3 m m

v pf

xd

vd

H H

BVPdMp

1

1

Thin lens

Changes in retinal image size

11

1

1

1

Pn

TBVPdSM

c

MpMp

MsMs

Spectacle magnification:

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

Changes in retinal image size

Spectacle magnification

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

Tc P1 P2 BVP SM

n = 1.523vd = 15

mmØ = 65 mm

OD 2.04 +5.21 -4.25 +1.00

7.1%OS 5.28 +7.78 -4.00 +4.00

n = 1.8vd = 12

mmØ = 65 mm

OD 1.66 +5.22 -4.25 +1.00

5%OS 3.68 +7.87 -4.00 +4.00

n = 1.8vd = 12

mmØ = 55 mm

OD 1.9 +5.22 -4.25 +1.00

3.8%OS 1.9 +5.22 -1.25 +4.00

Aniseikonia caused by anisometropia

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:Distortion in astigmatic lenses:

90º+ 2.00+ 4.00

+ 6.00

+ 4.00

+ 4.00

+ 6.00O BJ EC TE

Changes in retinal image size

min

max

SM

SM

max

minmax100SM

SMSMFAA

Changes in retinal image size

Angular distortion

O BJ EC TE

+ 6.00

+ 4.00

120º-2.00+6.00 D

Changes in retinal image size

Object

Image

Cylinder axis expressed as plus cylinder

Changes in retinal image size

Effect of angular distortion

+ 6 .0 0 D+ 4 .0 0 D

+ 6 .0 0 D+ 4 .0 0 D

Object 50º +2.00 +4.00 D lens

Changes in retinal image size

eff LL 212

2

22

1 1111

L

L

LL

L Pe

P

ePef

P

R= F’

2Lf

1Lf

e

Effect of vertex distance

Myope: Glasses prescription with vd of 12 mm, -9.00 D

Glasses prescription with vd of 15 mm, -9.25 D

1

12 1 Lo

LoLo Pe

PP

123

1215 1031 L

LL P

PP

Example:Example: New frameNew frame

Effect of vertex distance

12 dde

R=F’

LCf

1Lof

2Lof

dve

Effect of vertex distance

Example:Example: Fitting a contact lens (above 5.00 D)Fitting a contact lens (above 5.00 D)

Lo

LoLC Pdv

PP

·1

Movement of glasses to see

betterHyperopia Myopia

Closer to the eyes Overcorrected Undercorrected

Away from the eyes Undercorrected Overcorrected

Effect of vertex distance

Example:Example: SignsSigns

Example:

Signs

Effect of vertex distance