20 - Image Jump and Horizontal Prism at Near

Question 1

What causes Image Jump? 2

Answer 1

Crossing the border of the segment, prism amount suddenly changes.
Abrupt change in prismatic effect causes objects to suddenly be displaced.

Question 1

Practice finding image jump

Question 2

Where is the OC in a round seg?

Answer 2

The middle = half the segment measurement
22mm Rseg = OC is 11mm

Question 3

In a flat top segment how do you find the distance from the OC?

Answer 3

Seg depth - (Seg width/2) = distance from OC

Question 4

What 3 things determine the amount of accommodation needed to see at near?

Answer 4

-Near-viewing distance
-Power of the distance spec lens rx
-Distance from the lens to the principal planes of the eye.

Question 5

Who needs progressives first and why?

Answer 5

Hyperope as they have to accommdate almost an entire dioptre more at a 40cm distance compared to a myope

Question 6

What happens when add power increases?

Answer 6

The amount of accommodation needed between the hyperope and myope decreases.

Question 7

What do the Values in this Formula stand for?
Ld=1/(1/Fv’)-d (not on quiz)

Answer 7

Ld = the vergance of light at the corneal plane from infinity
FV’=the power of the lens
d=distance in meters from the reference position of the lens to the new reference position

Question 8

Why would an Optician change the add power? 3

Answer 8

Depending on the distance they need to see.
Closer = stronger add
Further away = weaker add

Question 9

Why do nonpresbyopes need a different cyl correction at near? 2

Answer 9

Large difference in powers = accommodate at different points
solves as px becomes presbyopic

Question 10

How much do these +7.00D/-7.00D/0.00D rx accommodate?
(Based on spec plane of 40.14cm)

Answer 10

Myope - +2.00D
Hyperope- +2.96D
Emmetrope- +2.42D

Question 11

How can you correct Horizontal prism at near? 5

Answer 11

1-2 pairs of glasses
2-Fresnell press on
3-Decentering Larger segment top lens
4-Split Lens
5-Cement segment construction

Question 12

Why would near prism be needed?

Answer 12

-If prism is needed for distance, different amount for near
-High Accommodative convergance
-Intermittent near tropias
-Non committal strabismus

Question 13

What are the 2 restrictions when decentering the segment for horizontal prism?

Answer 13

Too much decentration = lens size wont allow it
Segment too small = edge will intefer with distance vision

Question 14

If the segment is moved nasalward/temporally when decentering for horizontal prism, what is the base directions?

Answer 14

Nasally - Base In
Temporally - Base Out

Question 15

What is the seg inset formula?

Answer 15

Seg Inset= distance PD-NearPD/2

Question 16

What is the formula for finding seg size needed when correcting horizontal prism?

Answer 16

Seg size = 2(10+additional seg inset)

Question 17

What are the steps to verifying progressive lenses?

Question 18

What 2 things should we check when dispensing progressives?

Answer 18

-fitting cross should be in front of pupil centre
-horizontal dashes should be exactly horizontal and not tilted

Question 19

What are verifying measurements/markings in a progressive? 5

Answer 19

DRP
Fitting Cross
Horizontal Dashes
PRP
NRP

Question 20

What is the DRP? 2

Answer 20

Distance Reference Point
At the center of the arc

Question 21

Where does the fitting cross sit on the patient?

Answer 21

Ideally centered in the pupil

Question 22

What do the two horizontal dashes do?

Answer 22

Tell us if the lens is tilted or level

Question 23

What is the PRP? 4

Answer 23

-Prism reference point
-also known as major reference point (MRP)
-Centrally located
-used to verify prism power

Question 24

What is NRP? 3

Answer 24

-Near Reference Point
-circle in lower part of the lens
-used to verify near power