11.2 presbyopic additions

Question 1

What is spectacle amplitude?

Answer 1

maximum amplitude of accommodation measured in the spectacle plane

Question 2

Why is spectacle amplitude important?

Answer 2

relates to what we would prescribe in the spectacle plane to help in the cases of presbyopia but need to note that the change in the power needed at eye is different

Question 3

What happens if you are helping people focus at near when their accommodation is falling?

Answer 3

• it can not be possible to maintain maximum amplitude of accommodation
• Full amplitude of accommodation can not be sustained over a period of time
• muscular effort can lead to produce fatigue giving fluctuations in vision due to flucutations in accommodation and asthenopia- visual discomfort .

Question 4

What is possible to sustain of the amplitude of accommodation?

Answer 4

0. 5 - 2/3 of amplitude

- maintain half the maximum amp of accommodation

Question 5

How can we maintain 0.5 to 2/3 amplitude of accomodaiton?

Answer 5

• can re- plot the loss of accommodation with age curve to demonstrate the shorter focusing distance at different ages
• One curve - shows full amplitude of accomodation
• another curve - half amplitude of accommodation
• use a formula which is based on sustaining half amplitude
• this formula used to calculate the additional positive power for a presbyope

Question 6

Why is there full amplitude curve?

Answer 6

for reference

Question 7

What does the Half amplitude curve show?

Answer 7

can expect to maintain

Question 8

how can we calculate the additional positive power which can be helpful to a presbyopic patient ?

Answer 8

Add = -Ls - 1/2 Amp(s)

• depends on working distance required (target)
• based on amplitude of accommodation - half appears in the formula- as the formula based on sustaining half the spectacle amplitude of accommodation when looking at a target at that distances
• can change the value- willing to accept patient have 2/3rd of accommodation.- so can change it to 2/3.
• between 0.5 to 2/3

Question 9

What is where the target is?

Answer 9

working distance e

Question 10

What is the normal working distance?

Answer 10

ls = could be - 1/3m
giving Ls = -3D

-Usually between 380 to 540mm more common giving Ls = -2.5D

Question 11

Who can work at closer distances?

Answer 11

Those with defective acuity or involved in fine work may use closer distances

Question 12

What does the addition do?

Answer 12

allows eye to see image in sharp focus- sharpening effect
-allows object to be brought closer to eye and so subtends at a larger angle at the eye and therefore appears larger - ability to bring closer hence larger image formed

-Increased angular subtense when object brought closer provided subject can accommodate to bring it into focus.

Question 13

Why is the Amp o bigger than Amp s and how can you find that out?

Answer 13

• do this by calculating the Amp of accommodation of the spectacle plane and the ocular amplitude of accommodation
• due to the value of vergence at the eye

Question 14

What is the amplitude in the spectacle plane ?

Answer 14

is the power of the spectacle lens needed to make the object appear to come from infinity

Question 15

-Take our +5D corrected hypermetrope from the previous example.
-Assume the spectacle lens is pushed down his nose to make the vertex distance 30mm
Work out the effect it has by tracing vergences back- start from eye

Answer 15

• this is Forward spectacle shift-
• From the previous example:K = +5.32D

Ls’ = +4.59D
Ls = Ls’ – Fsp = -0.41D

Question 16

What is the forward spectacle shift of a positive spectacle lens - positive power in spectacle plane ?

Answer 16

it increases vergence at the eye and slightly decrease the distance at which you can see things clearly
-helps you see things closer more clearly.

Question 17

How can hypermetropes increase the effective positive power of the spectacle lens?

Answer 17

by pushing the lens down their nose-

-vertex distance increases, the positive verrgence of the eye increases and that allows them to see things closer

Question 18

How can increasing presbyopia be dealt with ?

Answer 18

-rather than having a new add in your prescription you can take the current add and push it down your nose to increase the positive power however doesnt work for myopes- they can simply remove their specs- as their far point is out infront of them

Question 19

Cylinder Effectivity at Near - looking at whether the astigmatic correction for distance is the same as the astigmatic correction for near ?

Answer 19

• if going to change the spherical component for near it could change the astigmatic correction.
  -So for distant vision - Distance Vision
  Analysing each principal meridian and finding K
  K150 = -2.87D
  K60 = -5.50D
  Ocular Astigmatism = 2.63D- less at eye than spectacle plane
  -For near vision (assume target 400mm)
  Along 150
  Ls’ = Ls + F 150 = -2.5 - 3 = -5.5D
  L2 = -5.08D
  Along 60
  Ls’ = Ls + F 60 = -2.5 - 6 = -8.5D
  L2 = -7.54D
  Ocular astigmatism = 2.46D

ocular astigmatism diff = 0.17D - the difference - could be better to look at cyl value of near to see di

Question 20

Why should you check for the cylinder in near?

Answer 20

-In high levels of astigmatim

could be better to look at cyl value of near to compare what you prescribe for distance.

Question 21

How much is possible amplitude to sustain of the accommodative amplitude?

Answer 21

0.5

Question 22

What is the normal working distance ?

Answer 22

400mm - but varies

Question 23

What is the effect of add on ocular accommodation found by ?

Answer 23

tracing vergences

Question 24

What can cylinder effectivity mean ?

Answer 24

can mean ocular astigmatism at near differs from that determined for distance refraction
-But RARE - only check in high levels of cyl / astigmatism .