Visual Acuity
Clinical Optometry 1
MOD007288
1
Learning Objectives
• Define and understand vision, visual
acuity, resolution acuity, minimum angle
of resolution.
• Understand the principles and use a
Snellen Chart.
• Understand the principles and use a
LogMAR Chart.
2
Learning Objectives
• Understand the factors affecting visual
acuity measurement.
• Be able to record visual acuity
appropriately.
• Be able to convert various measures of
visual acuity.
3
Vision and Visual acuity
• “Vision” (V) - Visual standard obtained
without a correction.
• No spectacles / contact lenses
• “Visual Acuity” (VA) - Visual standard
obtained with a correction.
• With glasses / contact lenses
4
Visual acuity
• Visual acuity describes the eye’s ability to
discriminate detail in an object
Visual acuity(VARes ) 
1

• ‘Res’ indicates that the acuity is resolution
acuity
• ω is angle expressed in arcminutes
5
The Minimum Angle of Resolution
• ω > MAR
S2
S1
• S1 and S2 are easily resolved
• ω < MAR
• S1 and S2 are not resolved
S1
S2
• S1 and S2 are just resolved
as separate
S1
S2
• ω = MAR
6
Factors determining resolution acuity
• Uncorrected refractive error which causes
defocus blur
• Image contrast
• Diffraction
• Aberrations
• Imperfections in the ocular media
• Imperfections of the eye’s refracting surfaces
• Back scatter from the fundus
7
Snellen Acuity
8
The Snellen letter chart
• An angular subtense of 1 arc minute
forms the basis for the construction of
the chart.
• This 1 minute of arc angular separation
is taken as the “norm” for a measure of
visual acuity using letters.
9
The Snellen Letter
• The limbs and spaces subtend
1 arc minute (a sixtieth of a
degree of angular
measurement) at specified
distances, and the letter height
subtends 5 arc minutes
• The original Snellen letter was
designed on a 5 x 5 grid and the
letters on the chart were serifed.
10
The Snellen letter (5 x 5)
5/
1/
11
The Snellen letter chart
• The Snellen letter chart consists of a
series of rows of letters whose height
subtends 5 arc minutes at distances of 6,
9, 12, 18, 24, 36 and 60 metres.
• The largest single letter is placed at the
top and the number of letters increases as
one moves down the chart.
12
Snellen Acuity
testing distance in meters
VA 
distance at which the height of letter subtends 5'
• This is known as the Snellen fraction or
Snellen acuity
13
Snellen Acuity
• Usual testing distance is 6m.
• If at that distance a patient just resolves a letter
that subtends an angle of 5’ at 6m, VA = 6/6
• The limbs of this letter subtend angle of 1’
• The MAR is 1’
14
6 metre letter
5’
6m
15
Why a testing distance of 6m?
• A testing distance of 6m produces an
incident vergence of -1/6 D(-0.17D) at the
eye
• -0.17D is substantially less than the 0.25D
prescribing interval
• 6m is therefore effectively infinity
16
Practicalities of 6m testing
• 6 metre requires a lot of space.
• Most testing rooms are 3 metres and
use a mirror to create a virtual image
6 metres away.
• Infinity boxes now common too
17
Short room testing
18
Recording the acuity - examples
• If the smallest letter a patient reads subtends
an angle of 5’ at a distance of 18m (the ‘18m’
letter), VA is recorded as 6/18
• If the patient can just resolve the ’12m’ letter,
VA = 6/12
• If the patient can just resolve the ’4m’ letter, VA
= 6/4
19
Recording the acuity
• The smallest row of letters which the patient can
read should always be recorded
• If a patient only reads part of a row this must also
be recorded as either:
• VA of the previous row read correctly plus (+)
“the number of letters from the following row
read correctly
or
• VA of the row that is being read minus (-) “the
number of letter that were read incorrectly”
20
Recording the acuity
• For example:
• If a patient reads all letters on the 6/12 line but
only 2 on the 6/9 line, this would be recorded
as 6/12+2
• If a patient reads 4 letters (out of 6) on the 6/9
line, this would be recorded as 6/9-2
21
Activity
• On the following two slides determine
what the Snellen acuity is:
22
60
Example Scoring Snellen
36
24
6/?
18
12
9
6
5
4
60
Example Scoring Snellen
36
24
6/?
18
12
9
6
5
4
LogMAR Acuity
25
logMAR chart
• Included in computer-based test charts.
logMAR charts
• Equal number of letters per line (5 letters per line)
• Letter spacing on each row is equal to one letter width.
The row spacing is equal to the height of the letters on
the next row.
• Constant change 0.1 log unit between rows of letters
logMAR Charts
• Each letter in a row is assigned a value of 0.02 log
unit
• Chart is not truncated at 6/6
• Results usually recorded as logMAR (where 6/6 =
logMAR 0)
• Chart can be used at different distances (half the
testing distance and add +0.30 to the logMAR
score)
28
Activity
• On the following two slides determine
what the logMAR acuity is:
29
Chart conversions
32
Factors affecting Testing Charts
33
Modern test charts
• Very few charts use the original Snellen
optotypes, but the principal of limb and letter
sizes remains the same and the charts are
referred to as Snellen
• Most modern charts use sans-serif upper case
letters constructed on a 5x5 or 5x4 grid
A sans-serif letter on a 5 x 4 grid
5/
1/
The design of modern test types
Factors that need to be considered:
• letter design and relative legibility
• progression of letter sizes
• letter and row spacing
• contrast between the letters and the background
• chart luminance
Letter design
• British standard BS 4274 -1: 2003 recommends
the following 5 x 5 sans-serif letters:
CDEFHKNPRUVZ
• No letters should be repeated on any one line
Progression of letter sizes
• Original Snellen chart:
• 6, 9, 12, 15, 21, 30 and 60
• Most modern charts:
• 5, 6, 9, 12, 18, 24, 36 and 60
• LogMAR charts use a 0.1 log unit interval
Letter and row spacing
• British Standards (BS 4274 -1: 2003) recommend
that:
• the gap between the letters should equal the width
of the letters
• lines of letters should be separated by a gap not
less than the letter height in the smaller line or 20
mm, whichever is the smaller
Letter contrast
• Black letters on a white background is an example of
a high contrast target while grey letter on a slightly
lighter background describes a low contrast target.
DEFNH
DEFNH
Letter contrast
• Contrast of a letter is described by Weber’s
definition:
• Contrast = Lmax-Lmin/Lmin
Lmax
Lmin
Letter contrast
• High contrast letters are easier to read than
those of low contrast
• The British Standards (BS 4274 -1: 2003)
recommendation is a minimum contrast of
0.9 (90%).
Chart luminance
• British Standards (BS 4274 -1: 2003) recommend
that the luminance should be uniform and not less
than 120 cd/m2
• The surround luminance level should be as near as
possible to the luminance level as the background on
the chart.
Chart distance
• Current British Standards (BSI 4274-1 2003)
state that the testing distance should be no less
than 4m.
Design flaws of the Snellen chart
• Some letters are easier to recognise than others.
• Only one letter at 6/60 (limited use with low vision
px!)
• Truncation at 6/6
• Different numbers of letters on each line (more
crowding on smaller letters and more difficult to read
smaller lines)
• No systematic relationship between the spacing
between each letter and each row of letters.
Other Letter Charts
46
The Landolt ring and Tumbling E
• Four alternative forced choice test
• Landolt ring (C) -patient is asked to state the position
of the gap: up, down, right or left
• Tumbling E – patient is asked to the direction the E is
pointing: up, down, right or left
The Landolt ring and Tumbling E
5'
1'
1' 5'
Other charts
Lea symbols logMAR chart
Elliott, D.B. 2020. Clinical Procedures in Primary Eye Care 5th ed: Elsevier.
Recording Visual Acuity
50
Recording Visual Acuity
• Snellen (metres)
• Snellen (feet)
• Decimal
• MAR
• logMAR
51
Converting Snellen (metres) to
Snellen (feet)
• 6/6 = 20/20
• Numerator will always be 20
• Multiply denominator by 10/3
• E.g., 6/24 = 20/(24x10/3) = 20/80
52
Converting Snellen (metres) to
Decimal
• 6/6 = 1.0
• Divide the numerator by the
denominator.
• E.g., 6/24 = 0.25
53
Converting Snellen (metres) to MAR
• 6/6 = 1 MAR
• Divide the denominator by the
numerator.
• 6/24 = 24/6 = 4
54
Converting Snellen (metres) to
logMAR
• 6/6 = 0.00
• Get the log of the MAR.
• E.g., 6/24
• MAR of 6/24 is 4 (previous slide)
• Log4 = 0.60
55
Activity
• The smallest line of letters that a patient
reads at 6m is a row labelled 18 on the
chart.
• Record the visual acuity, as a Snellen
fraction (m and feet) and in decimal
notation. What is the MAR? logMAR?
Activity
Reads a row labelled 18 on the
chart
• Fraction: VA = 6/?
• Decimal: VA = ?
• MAR = 1/VA = ?
• logMAR = ?
• Fraction (feet): VA = 20/?
• Fraction: VA = 6/18
• Decimal: VA = 0.33
• MAR = 3’
• logMAR = 0.47
• Fraction (feet): VA = 20/60
Comparison of VA notations
MAR (')
Decimal
acuity
Snellen Snellen logMAR
acuity at acuity at
6m
20 feet
6/12
1.0
20/200
Comparison of VA notations
MAR (')
Decimal
acuity
0.50
Snellen
acuity at
6m
6/12
Snellen logMAR
acuity at
20 feet
20/40
0.30
2
1
1.0
6/6
20/20
0.00
10
0.10
6/60
20/200
1.00
Recording the acuity
• If a patient’s vision is less that 6/60, this must also be recorded.
• Ideally you would have a specialist chart for this – e.g., BRVT
• 3m? 1m? CF? HM? LP?
• 60 m letter read at 3 m
• 60 m letter read at 1 m
• Counts fingers at 1m
• Hand movements
• Light perception
• No light perception
3/60
1/60
CF @ 1m
HM
LP
NLP