how does retinoscopy work?

Question 1

myopia = against movement

Answer 1

• the far point is in front of the eye
• retinoscopy light forms an image S′ at that far point
• so when you tilt the retinoscope:
• the facial light patch moves in one direction (e.g., down)
• but the far point image S′ moves in the opposite direction
  → causing AGAINST movement

Question 2

hyperopia = with movement

Answer 2

• far point is virtual and behind the retina
• retinoscopy light forms a virtual image S′ behind the eye
• when the retinoscope is tilted:
• facial light patch moves one way
• far point image S′ moves the same way
  → WITH MOVEMENT (same direction)

Question 3

emmetrope = no movement

Answer 3

• light from retina produces parallel rays (far point at infinity).
  Retinoscope sees full pupil filled, no directional motion.
  Apparent motion is infinitely fast, so movement appears absent.

Question 4

high myopia

Answer 4

• dim reflex
• slow movement
• far point is very close to the eye
• light has diverged a
  lot by the time it
  reaches the sighthole
  and only a small
  portion enters
• Retinoscope has to be ‘moved a lot’ to get
  out of the cone of reflected light
• greater the myopia, the closer to the eye the image is formed

Question 5

low myopia

Answer 5

• bright reflex
• fast movement
• far point farther away
• less divergence → more light reaches retinoscope → brighter reflex

Question 6

high hyperopia

Answer 6

far point far behind retina → strongly divergent → dim + slow

Question 7

low hyperopia

Answer 7

far point nearer → bright + fast

Question 8

neutral

Answer 8

• BRIGHTEST REFLEX
  NO MOVEMENT/INFINITELY FAST
• S′ sits exactly at the retinoscope
• light appears to return from the same location as the retinoscope.
• moving the retinoscope moves the reflex equally → movement appears instantaneous → seen as NO MOVEMENT.

Question 9

what is S’?

Answer 9

S’ is the image of S which is formed at the far point of the eye.

Question 10

what is S?

Answer 10

Patch of light on the retina S, acting as a “source” of light

Question 11

What does K stand for?

Answer 11

the refractive error

Question 12

what does V stand for?

Answer 12

Relationship between the speed of the reflex

Question 13

Speed–Refractive Error Relationship

Answer 13

• speed decreases with increasing refractive error (K)
• At neutral: reflex speed = infinite

Question 14

vergence control

Answer 14

• retinoscopes have a slide that
  moves the condensing lens up
  and down
• slide at bottom = divergent beam (normal way), (IS just behind the retinoscope)
• slide at top = convergent beam (IS between the retinoscope and
  the patient)

Question 15

why might you have the slide at the top?

Answer 15

• brighter reflex
• ## useful in: high refractive index, locating cylinder axis more precisely, when initial reflex is hard to see