(2) Snellen Vs LogMar

Question 1

State snellen fraction

Answer 1

Testing distance / distance where optotype would subtend 5 min of arc at retina

Question 2

Angular subtense (min of arc)

Answer 2

Reciprocal of snellen fraction * 5

Question 3

Formula to find letter size (remember ‘E’ has five fingers)

Answer 3

Tan(min of arc / 60) = height of letter / 6m

• remember to multiply 1000 for height of letter in mm

Question 4

instructions for px

Answer 4

look at the chart, read the smallest you can see

Question 5

What to do when VA worse than 6/60 or 6/120

Answer 5

Move patient / chart nearer half distance each time

2 or 3 lines at least be seen

Question 6

What to do when VA worse than 1/60 or 1/120

Answer 6

1. Counting fingers - CF @ 1m
2. Hand movement - HM @ 1m
3. Perception of light - PL with projection, PL only, NPL

Question 7

3 Inadequacies in snellen

Answer 7

• Letter legibility inconsistent (6/9 +2, 6/36-2)
• Letter size non linear progression (6/6 to 6/9 to 6/36)
• Inconsistent no. of letters in a line

Question 8

How is LogMAR system expressed

Answer 8

-logarithm of minimum angle of resolution

Subtense _ min of arc = MAR
Logmar = [log] [MAR]

Question 9

4 LogMAR design

Answer 9

• Linear logmar letter progression size (line 0.1 letter 0.02, avoids +/- sign use)
• same numbers of optotype per line (5 letters)
• Letters similar legibility and per letter scoring
• simple calculation (log scale)

Question 10

What to do when VA worse than 1.0 in LogMAR

Answer 10

Move patient / chart nearer by half distance (0.3 to final value)

Question 11

Snellen to Logmar conversion

Answer 11

• MAR: divide snellen’s denominator by numerator
• Compute log
• better VA, lower LogMAR score (-ve if better than 6/6)

Question 13

what to look out for/observations

Answer 13

ensure patient is not leaning, squinting, tilting