Visual Resolution

Question 1

What is the Snellen fraction?

Answer 1

Test distance over the distance at which a letter subtends 5 min of arc

Question 2

Describe the conversion of degree to minute it second.

Answer 2

1 degree= 1/360 of a circle
1 minute = 1/60 degree
1 second= 1/60 minute

Question 3

Describe recognition acuity.

Answer 3

You need to be able to see the letter and name them to pass the test

Question 4

Describe detection acuity.

Answer 4

Observing if a patient look towards an image indicating that they can see it.
Useful for infants when recognition acuity is not possible

Question 5

Describe resolution acuity.

Answer 5

Tumbling E or Landolt C (each letter has same amount of difficulty)
Patient only need to be able to see the target

Question 6

Describe localization acuity and vernier acuity.

Answer 6

The smallest gap between the lines that the patient can resolve

The smallest angle between the line that patient can resolve

Question 7

Describe the relationship between viewing distance and angular subtense.

Answer 7

When you halve viewing distance, angular subtense of letter doubles

Double viewing distance, the angular subtense of the letter halves

Ex: 1/20th of distance, the letter will subtend 20x the angle

Question 8

Describe the LogMAR chart.

Answer 8

5 letter on every line
Each line is 0.10 logMAR less than the previous line
Space between each letter is equal to the width of the letter
Space between each line is equal to the height of the letters in the lower line
Each letter 0.02 (every letter counts, subtract from total)
Lower score is better (start at 0.50)

Question 9

What is normal V/A

Answer 9

20/20 is NOT normal VA in a healthy individual all the way up to 75 years of age

Question 10

Does MAR change with viewing distance?

Answer 10

No

Question 11

What can we do if the patient cannot read the top letter on the chart?

Answer 11

Move the chart or patient closer
Hand motion
Light projection
Light perception
No light perception

Question 12

Describe the Jaeger chart for measuring near VA.

Answer 12

Letters range from J1 to J20
Not standardized

Question 13

Describe the M notation near V/A chart.

Answer 13

M denotes the distant in meters which the letter subtends 5 minutes of arc (denominator of Snellen fraction)

Question 14

Describe the point N near VA chart.

Answer 14

One point is 1/72 of an inch
(N5= 5/72 inches high)
Commonly used in computer fonts
Refers to size of block not letter

Question 15

What 3 factors affect V/A?

Answer 15

Blur
Constrast
Retinal eccentricity

Question 16

How can blur due to blur circle be reduced?

Answer 16

Reducing diameter of the blur circle with a pinhole aperture

Question 17

What is the function of a pinhole?

Answer 17

Reduce diameter of blur circle no where the light focuses (same refractive error stays)

If pinhole does not help, another issue is present

Question 18

How can we calculate contrast on a Snellen letter?

Answer 18

Contrast = (Lmax - Lmin) / (Lmax + Lmin)

Lmax- white
Lmin- black

Question 19

What happens to V/A as contrast decreases?

Answer 19

V/A will decline with decreasing contrast
Increasing contrast once adequate contrast has been achieved will not improve visual resolution in normal, healthy eyes

Question 20

What is the retinal eccentricity for V/A of 20/20, 20/30, 20/40 and 20/60?

Answer 20

Fovea - 20/20
1 degree - 20/30
1.5-2 degree - 20/40
3-5 degree - 20/60