1.5 - LogMAR chart

Question 1

Advantages of Snellen Charts

Answer 1

Quick
Reliable
Sensitive to ametropia ( as we develop higher levels of ammetropia = we lose lines on snellen chart more quickly than other measurement method )
Narrow (physical width)
Less affected by guessing compared to grating acuity, Landolt C or illiterate E charts
Letters familiar to most
Enshrined in legislation (at least certain lines)

Question 2

Limitations of Snellen Chart

Answer 2

Unequal number of letters per line – task is not equivalent for differing levels of VA ( different tasks at different lines/VA/vision ) e.g. 6/36 = task is 2 letters but 6/3 is 8 letters, ideally you want the same
Non-uniform change in letter size for successive lines
Line by line scoring system ( not single quantitative measure of someones vision - not true letter scoring system )
Fixed row spacing ( want equal spacing around letters, same at all levels - letters look closer together at top than bottom
Unequal letter spacing – crowding effects change - varies down chart.

Question 3

What can amblyopia be affected by

Answer 3

Crowding - ability to recognise a letter when there are surrounding features = there’s interaction between those surrounding features and letter trying to identify

Question 4

What was the 1st type of logMAR chart

Answer 4

Bailey and Lovie (1976) chart

Question 5

What did Bailey and Lovie (1976) chart do

Answer 5

Got rid of limitations in snellen chart

Question 6

Whats special about logMAR chart - number of letters

Answer 6

Same number of letters per line

Question 7

How many letters on each line in logMAR chart

Answer 7

5

Question 8

Whats special about legibility on logMAR chart

Answer 8

Similar legibility

Question 9

How is logMAR chart reliable

Answer 9

• Restrictive range of letters used
• Similar range of letters used
• (Approx) Equivalent task at each level of VA -

Question 10

What is letters size progression on snellen chart

Answer 10

Not a fixed ratio
Not even size progression down chart
Ratio between rows and measure of size of rows change - no fixed ratio on snellen chart - uneven progression

e.g. 6/3 to 6/4 = 1.33
6/4 to 6/5 = 1.25

Question 11

What is letters size progression on logMAR chart

Answer 11

For logMAR common ratio is chosen as
This is 0.1 log unit = 1.2589
Fixed ratio, logarithmic scale, even progression
Even steps between lines
Additional lines to fill spacing between the lines and give fixed ratio which gives even progression and steps- NOT on snellen chart

Question 12

LogMAR Scoring

Answer 12

0.1 log unit per line ( size changes by 0.1 log unit from 1 line to next down chart )
Therefore 0.02 logMAR units per letter since 5 letters per line
Size halves/doubles every 3 lines

Question 13

Letter Spacing on logMAR chart

Answer 13

Letter spacing on each line = letter width ( same )

Equivalent to 4 limb widths ( gap between letters ) since 5 x 4 letters on a Bailey-Lovie chart

Question 14

Line Spacing on logMAR chart

Answer 14

Height of letters in the smaller row EQUAL width of letters in the larger row
Might look like lines getting closer together down chart but because its fixed to height of letters - constant

Question 15

Summary of snellen chart

Answer 15

Unequal number of letters per line 
Non-uniform change in letter size for successive lines
Line by line scoring system
Fixed row spacing
Unequal letter spacing

Question 16

Summary of logMAR chart

Answer 16

5 letters per line
0.1 log unit (1.2589) change in letter size per line
Letter scoring system (0.02 per letter)
Height of letters in the smaller row
Letter width