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SUBJECTIVE
REFRACTION
OPTOM FASLU MUHAMMED
REFRACTION PROCEDURE: DISTANCE
2
1. Visual acuity (VA) with current Rx
VA versus spherical Rx table
• Spectacles
• AR
• Retinoscope
• Unaided
2. Perform Pinhole test
Unaided VA with existing Rx if aided VA is
worse than 6/12
REFRACTION PROCEDURE: DISTANCE
3. Determine Best Vision sphere
3
• Letter Chart
• Duochrome
4. Estimate Astigmatism
• VA with BVS
• Refractive Error Estimation table
4
5. Refine Astigmatism (cylinder)
• Axis location
• Astigmatic power
- Fan & Block
- Jackson X-cyl
- Cyl trial lens Rotation & Letter
chart
6. Refine Sphere of Rx
• +1.00 DS Fog to blur vision
Refraction Procedure: Distance
REFRACTION PROCEDURE: DISTANCE
5
7. Bi-Ocular balance
• Simultaneous
• Alternating
- Prism Dissociation
- Polaroid
8. Final
Rx• Option
s
REFRACTION PROCEDURE: NEAR ADD
1. Establish Preferred Near Viewing Distance
6
2. Determine Near Blur Point
• Binocular
• Monocular recheck
– near letter chart
– measuring tape
• Binocular
– measuring
tape
REFRACTION PROCEDURE: NEAR ADD
3. Calculate Near ADD required
7
• Maintain half (50%) the Amplitude of
Accommodation in reserve
- nomogram, nomograph (in the workbook)
4. Refine ADD
• Range of clear vision
- near letter chart
- trial lens
Subjective refraction
LOGMAR CHARTS
•The design principles
suggested by Bailey & Lovie
•More lines than a typical
Snellen chart, particularly at
poor VA levels
•Not truncated to 6/5 (20/15)
or similar.
TYPES
 ADULTS
– Bailey–Lovie Charts
– ETDRS charts
 CHILDREN
– The Glasgow Acuity Cards
 Keeler crowded logMAR charts
PROCEDURE
 The luminance of the chart should be between 80 and
320cd/m2.
 Seat the patient comfortably with an unobstructed view of
the test chart.
 You should sit in front and to one side of the patient in
order to monitor facial expressions and reactions.
Subjective refraction
•They should be pushed to
determine whether they can see
any more.
•If they make four or more
mistakes on a line of five
•Ask them to move closer if chart
not seen
If the patient cannot see the letters
even at the closest test distance
a) Hand Movements (HM) @ Y
cm
b) Light Projection (Lproj.):50 cm
c) Light Perception (LP):
Repeat measurements for the other
eye and binocularly.
Since each letter is
worth an equal 0.02
log units, you can
most accurately
determine visual acuity
by accounting for every
letter correctly identified.
Typically, an equation is
used to calculate VA:
logMAR VA = 1.1 - (0.02 x letters missed)
MOST COMMON ERRORS
1. Allowing cautious patients to decide their acuity (i.e. not pushing them to
guess).
2. Permitting the patient to screw their eyes up and improve their VA.
3. Permitting the patient to look around the occluder or through their fingers
and view binocularly when measuring monocular VA.
4. Taking distance VAs in a PAL or varifocal wearer when they are not looking
through the distance vision section of the lens.
5. Using an incorrect working distance.
6. Not recording the result immediately and guessing the result at the end of
the examination.
VA MEASUREMENT USING
SNELLEN CHART
 Procedure is similar to logMAR chart
 Vn is recorded as the smallest line in which the
majority of letters are seen,irrespective of
subjective blurr.
 Errors are recorded by appending a -1,-2, or -3 to
snellen fraction
Eg:6/9(-2),6/9(-3)…
If pt couldn’t see 6/60 letter @ 6m,but could @
2m,recorded as 2/60.
RECORDING
 The Snellen fraction is defined as:
 Test Distance
/Distance at which the letters subtend 5 min of arc.
1. Test distance can be provided in metres (metric) or
feet (imperial).
1. Snellen VA can be labelled in either decimal or
conventional Snellen notation
Snellen Fraction = Test Distance
Distance at which entire letter subtends 5’
The acuity test distance should be long enough to not stimulate the
accommodative system. By convention, the standard test distance in the
U.S. has been 20 feet. Everywhere else it is 6 meters:
20 is a Snellen fraction in feet
60
6 is the same Snellen fraction
18 in metric units
Measuring visual acuity: The Snellen Fraction
Measuring Distance Visual Acuity
Testing Sequence
By convention you always test in the following order
1st OD ocular dexter Right eye (cover left eye)
2nd OS ocular sinister Left eye (cover right eye)
3rd OU ocular utrique Both eyes
Measuring Distance Visual Acuity: Example 2
A P E O T F 20/25
E V O C T Z 20/20
O H P N T C 20/15
What is the visual acuity of this patient?
5/6 letters on the 20/25 line and only 2 letters on the 20/20 line
No reason to even test the 20/15 line
The visual acuity can be expressed as 20/25
However, accounting for the missed and identified letters...
The VA can be more descriptively expressed as 20/25-1/+2
MONOCULAR SUBJECTIVE RX
Letter Chart
Duochrome
test
Combination
22
Spherical Correction
= Best vision sphere (BVS)
Obtain spherical correction giving best VA
MONOCULAR SUBJECTIVE RX
Starting point for BVS
 Sphere component of Spec Rx
 Sphere component of Auto-Rx
 Unaided VA Estimation Method
23
BVS = (Sphere Rx) + (Astigmatic Rx ÷ 2)
Best Vision Sphere (BVS)
VISUAL ACUITY
Snellen 6/VA Decimal MAR (Minutes of Arc) logMAR
3 2.00 0.50 -0.30
4 1.50 0.67 -0.18
5 1.20 0.83 -0.08
6 1.00 1.00 0.00
7.5 0.80 1.25 0.10
9 0.67 1.50 0.18
9.5 0.63 1.58 0.20
12 0.50 2.00 0.30
15 0.40 2.50 0.40
18 0.33 3.00 0.48
19 0.32 3.17 0.50
24 0.25 4.00 0.60
30 0.20 5.00 0.70
36 0.17 6.00 0.78
38 0.16 6.33 0.80
48 0.13 8.00 0.90
60 0.10 10.00 1.00
120 0.05 20.00 1.30
24
RELATIONSHIP BETWEEN REFRACTIVE ERROR
AND VA
25
Vision
6/6 (20/20)
6/9 (20/30)
6/12 (20/40)
6/18 (20/60)
6/24 (20/80)
6/36 (20/120)
6/60 (20/200)
Spherical*
small
0.50
0.75
1.00
1.50
2.00
2.00 to 3.00
Astigmatic
small
1.00
1.50
2.00
3.00
4.00
high
Refractive Error (D)
Bennett and Rabbetts, 1984
* Myopia or absolute hypermetropia
RELATIONSHIP BETWEEN REFRACTIVE ERROR
AND VA
26
Vision
6/6 (1.00)
6/9 (0.67)
6/12 (0.50)
6/18 (0.33)
6/24 (0.25)
6/36 (0.13)
6/60 (0.10)
Spherical *
< 0.50
0.50
0.75
1.00
1.50
2.00
2.00 to 3.00
Astigmatic
< 0.75
1.00
1.50
2.00
3.00
4.00
> 4.00
Refractive Error (D)
Bennett and Rabbetts, 1984 * Myopia or absolute hypermetropia
ESTIMATE RESIDUAL SPHERICAL ERROR:
DISTANCE RX
27
• Note the Best Snellen VA obtained
• Estimate the Amount of Spherical
Error
Procedure
Residual Spherical Error =
Snellen (metric) VA Denominator ÷16
ESTIMATE RESIDUAL SPHERICAL ERROR:
DISTANCE RX
Unaided Visual
Acuity
RE LE
Patient A
Spherical Rx
Snellen 6/12 Snellen 6/9
Patient B
Spherical Rx
Snellen 20/30 Snellen 20/60
Patient C
Spherical Rx
Decimal VA 0.1 Decimal VA 0.17
Patient D
Spherical Rx
MAR 4 logMAR 0.7 28
0.75D 0.56D
0.56D 1.12D
2.25D3.75D
1.50D 1.75D
MONOCULAR SUBJECTIVE RX
• Use the ‘highest plus’ sphere as a starting
point
• Add plus lenses in 0.50 steps until VA
begins to decrease
• Then add negative lenses in 0.25D steps
until there is no further VA improvement:
“Does this lens make the letters clearer
or just smaller and darker?”
29
Refinement: Sphere (BVS)
Letter Chart
SPHERICAL CORRECTION
Points to note:
 Accommodation
 if the letters get darker and smaller only,
then too much minus has been
prescribed
 When adding plus lenses,
assure the patient you know it is
getting worse
30
Letter Chart
SPHERICAL CORRECTION
Duochrome
(starting point)
31
Combination Tests
Letter chart
(refinement)
Duochrome
(end point)
Letter chart
(BVS)
OR

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Subjective refraction

  • 2. REFRACTION PROCEDURE: DISTANCE 2 1. Visual acuity (VA) with current Rx VA versus spherical Rx table • Spectacles • AR • Retinoscope • Unaided 2. Perform Pinhole test Unaided VA with existing Rx if aided VA is worse than 6/12
  • 3. REFRACTION PROCEDURE: DISTANCE 3. Determine Best Vision sphere 3 • Letter Chart • Duochrome 4. Estimate Astigmatism • VA with BVS • Refractive Error Estimation table
  • 4. 4 5. Refine Astigmatism (cylinder) • Axis location • Astigmatic power - Fan & Block - Jackson X-cyl - Cyl trial lens Rotation & Letter chart 6. Refine Sphere of Rx • +1.00 DS Fog to blur vision Refraction Procedure: Distance
  • 5. REFRACTION PROCEDURE: DISTANCE 5 7. Bi-Ocular balance • Simultaneous • Alternating - Prism Dissociation - Polaroid 8. Final Rx• Option s
  • 6. REFRACTION PROCEDURE: NEAR ADD 1. Establish Preferred Near Viewing Distance 6 2. Determine Near Blur Point • Binocular • Monocular recheck – near letter chart – measuring tape • Binocular – measuring tape
  • 7. REFRACTION PROCEDURE: NEAR ADD 3. Calculate Near ADD required 7 • Maintain half (50%) the Amplitude of Accommodation in reserve - nomogram, nomograph (in the workbook) 4. Refine ADD • Range of clear vision - near letter chart - trial lens
  • 9. LOGMAR CHARTS •The design principles suggested by Bailey & Lovie •More lines than a typical Snellen chart, particularly at poor VA levels •Not truncated to 6/5 (20/15) or similar.
  • 10. TYPES  ADULTS – Bailey–Lovie Charts – ETDRS charts  CHILDREN – The Glasgow Acuity Cards  Keeler crowded logMAR charts
  • 11. PROCEDURE  The luminance of the chart should be between 80 and 320cd/m2.  Seat the patient comfortably with an unobstructed view of the test chart.  You should sit in front and to one side of the patient in order to monitor facial expressions and reactions.
  • 13. •They should be pushed to determine whether they can see any more. •If they make four or more mistakes on a line of five •Ask them to move closer if chart not seen
  • 14. If the patient cannot see the letters even at the closest test distance a) Hand Movements (HM) @ Y cm b) Light Projection (Lproj.):50 cm c) Light Perception (LP): Repeat measurements for the other eye and binocularly.
  • 15. Since each letter is worth an equal 0.02 log units, you can most accurately determine visual acuity by accounting for every letter correctly identified. Typically, an equation is used to calculate VA: logMAR VA = 1.1 - (0.02 x letters missed)
  • 16. MOST COMMON ERRORS 1. Allowing cautious patients to decide their acuity (i.e. not pushing them to guess). 2. Permitting the patient to screw their eyes up and improve their VA. 3. Permitting the patient to look around the occluder or through their fingers and view binocularly when measuring monocular VA. 4. Taking distance VAs in a PAL or varifocal wearer when they are not looking through the distance vision section of the lens. 5. Using an incorrect working distance. 6. Not recording the result immediately and guessing the result at the end of the examination.
  • 17. VA MEASUREMENT USING SNELLEN CHART  Procedure is similar to logMAR chart  Vn is recorded as the smallest line in which the majority of letters are seen,irrespective of subjective blurr.  Errors are recorded by appending a -1,-2, or -3 to snellen fraction Eg:6/9(-2),6/9(-3)… If pt couldn’t see 6/60 letter @ 6m,but could @ 2m,recorded as 2/60.
  • 18. RECORDING  The Snellen fraction is defined as:  Test Distance /Distance at which the letters subtend 5 min of arc. 1. Test distance can be provided in metres (metric) or feet (imperial). 1. Snellen VA can be labelled in either decimal or conventional Snellen notation
  • 19. Snellen Fraction = Test Distance Distance at which entire letter subtends 5’ The acuity test distance should be long enough to not stimulate the accommodative system. By convention, the standard test distance in the U.S. has been 20 feet. Everywhere else it is 6 meters: 20 is a Snellen fraction in feet 60 6 is the same Snellen fraction 18 in metric units Measuring visual acuity: The Snellen Fraction
  • 20. Measuring Distance Visual Acuity Testing Sequence By convention you always test in the following order 1st OD ocular dexter Right eye (cover left eye) 2nd OS ocular sinister Left eye (cover right eye) 3rd OU ocular utrique Both eyes
  • 21. Measuring Distance Visual Acuity: Example 2 A P E O T F 20/25 E V O C T Z 20/20 O H P N T C 20/15 What is the visual acuity of this patient? 5/6 letters on the 20/25 line and only 2 letters on the 20/20 line No reason to even test the 20/15 line The visual acuity can be expressed as 20/25 However, accounting for the missed and identified letters... The VA can be more descriptively expressed as 20/25-1/+2
  • 22. MONOCULAR SUBJECTIVE RX Letter Chart Duochrome test Combination 22 Spherical Correction = Best vision sphere (BVS) Obtain spherical correction giving best VA
  • 23. MONOCULAR SUBJECTIVE RX Starting point for BVS  Sphere component of Spec Rx  Sphere component of Auto-Rx  Unaided VA Estimation Method 23 BVS = (Sphere Rx) + (Astigmatic Rx ÷ 2) Best Vision Sphere (BVS)
  • 24. VISUAL ACUITY Snellen 6/VA Decimal MAR (Minutes of Arc) logMAR 3 2.00 0.50 -0.30 4 1.50 0.67 -0.18 5 1.20 0.83 -0.08 6 1.00 1.00 0.00 7.5 0.80 1.25 0.10 9 0.67 1.50 0.18 9.5 0.63 1.58 0.20 12 0.50 2.00 0.30 15 0.40 2.50 0.40 18 0.33 3.00 0.48 19 0.32 3.17 0.50 24 0.25 4.00 0.60 30 0.20 5.00 0.70 36 0.17 6.00 0.78 38 0.16 6.33 0.80 48 0.13 8.00 0.90 60 0.10 10.00 1.00 120 0.05 20.00 1.30 24
  • 25. RELATIONSHIP BETWEEN REFRACTIVE ERROR AND VA 25 Vision 6/6 (20/20) 6/9 (20/30) 6/12 (20/40) 6/18 (20/60) 6/24 (20/80) 6/36 (20/120) 6/60 (20/200) Spherical* small 0.50 0.75 1.00 1.50 2.00 2.00 to 3.00 Astigmatic small 1.00 1.50 2.00 3.00 4.00 high Refractive Error (D) Bennett and Rabbetts, 1984 * Myopia or absolute hypermetropia
  • 26. RELATIONSHIP BETWEEN REFRACTIVE ERROR AND VA 26 Vision 6/6 (1.00) 6/9 (0.67) 6/12 (0.50) 6/18 (0.33) 6/24 (0.25) 6/36 (0.13) 6/60 (0.10) Spherical * < 0.50 0.50 0.75 1.00 1.50 2.00 2.00 to 3.00 Astigmatic < 0.75 1.00 1.50 2.00 3.00 4.00 > 4.00 Refractive Error (D) Bennett and Rabbetts, 1984 * Myopia or absolute hypermetropia
  • 27. ESTIMATE RESIDUAL SPHERICAL ERROR: DISTANCE RX 27 • Note the Best Snellen VA obtained • Estimate the Amount of Spherical Error Procedure Residual Spherical Error = Snellen (metric) VA Denominator ÷16
  • 28. ESTIMATE RESIDUAL SPHERICAL ERROR: DISTANCE RX Unaided Visual Acuity RE LE Patient A Spherical Rx Snellen 6/12 Snellen 6/9 Patient B Spherical Rx Snellen 20/30 Snellen 20/60 Patient C Spherical Rx Decimal VA 0.1 Decimal VA 0.17 Patient D Spherical Rx MAR 4 logMAR 0.7 28 0.75D 0.56D 0.56D 1.12D 2.25D3.75D 1.50D 1.75D
  • 29. MONOCULAR SUBJECTIVE RX • Use the ‘highest plus’ sphere as a starting point • Add plus lenses in 0.50 steps until VA begins to decrease • Then add negative lenses in 0.25D steps until there is no further VA improvement: “Does this lens make the letters clearer or just smaller and darker?” 29 Refinement: Sphere (BVS) Letter Chart
  • 30. SPHERICAL CORRECTION Points to note:  Accommodation  if the letters get darker and smaller only, then too much minus has been prescribed  When adding plus lenses, assure the patient you know it is getting worse 30 Letter Chart
  • 31. SPHERICAL CORRECTION Duochrome (starting point) 31 Combination Tests Letter chart (refinement) Duochrome (end point) Letter chart (BVS) OR