9 - Prism in Multifocals - Exam 2

Question 1

Does image jump depend on the distance power or the distance OC location?

Answer 1

No

Only depends on near OC and add power

Question 2

What is the equation for image jump?

Answer 2

(Seg power)(distance from seg top to seg OC) = image jump

Question 3

What is another phrase for object displacement?

Answer 3

Total prismatic effect

Question 4

What 4 pieces of information must be given to compute object displacement?

Answer 4

1. Distance OC in relation to segment
2. Seg drop
3. Seg OC
4. Location of NRP

Question 5

What is the relationship between seg inset and distance Rx?

Answer 5

More minus distance Rx - smaller seg inset

More plus distance Rx - larger seg inset

Question 6

What is anisometropia?

Answer 6

Significant difference in power between eyes

Question 7

What is anisophoria?

Answer 7

Unequal prismatic effects resulting from looking away from the optical centers of anisometropic lenses

Question 8

Patients who have convergence problem need what kind of prism at near?

Answer 8

Base in prism

Question 9

What are 3 conditions that would indicate a patient needs near prism?

Answer 9

1. Too high or too low of AC/A ratio
2. Periodic strabismus at near
3. Nonconcomitant strabismus

Question 10

What are 4 ways to obtain horizontal prism at near?

Answer 10

1. Two separate pairs of glasses (one for near, one for far)
2. Stick-on Fresnel prism
3. Prism segment bifocals (very noticeable)
4. Increase the seg inset

Question 11

Where is unequal prism more troublesome, in the horizontal or vertical direction?

Answer 11

Vertical direction

Question 12

What is VPI?

Answer 12

Vertical Prism Imbalance

Difference in vertical prism between the two eyes

Question 13

Who will most likely experience VPI?

Answer 13

An anisometropic presbyope

Question 14

How much of a difference in Rx is generally tolerated in terms of VPI?

Answer 14

Up to 1.50 D of anisometropia is tolerated

Question 15

What is the only thing that VPI depends on?

Answer 15

The distance Rx

Question 16

What are 7 ways to correct VPI?

Answer 16

1. Contact lenses
2. Two separate pairs of glasses
3. Drop the distance optical center
4. Raise the seg height
5. Fresnel prism
6. Dissimilar segs
7. Slab off lens

Question 17

How far below the geometrical center should the optical center be in a near vision pair of glasses?

Answer 17

5 mm lower

Question 18

What type of prism is removed in a slab off lens?

Answer 18

Base down prism removed

Question 19

Which lens does slab off occur in?

Answer 19

The most minus (or least plus) lens with respect to the vertical meridian

Question 20

What is the name of the process for slab off lenses?

Answer 20

Bicentric grinding

Question 21

Where is the slab taken off for plastic lenses?

Answer 21

The back surface

Question 22

Where is the slab taken off for glass lenses?

Answer 22

The front surface

Question 23

What is reverse slab off?

Answer 23

Base down ADDED to most plus (or least minus) lens

Question 24

What is the equation for calculating the vertical distance between seg centers for dissimilar segs?

Answer 24

Z = (c) x (ADD)