< Page 17 of 19 >

1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Multifocal Designs
More Practice Exercises

In the following prescriptions, calculate the amount of slab off needed to neutralize the vertical imbalance at the reading level. In all instances consider the the reading level to be 10 mm from the distance optical center.

Example: Rx: � 1.75 � 0.50 x 180 � 4.75 � 0.25 x 180 Add: + 2.00
Step 1: Determine power in 90� meridian for each lens.
R: � 2.25 L: � 5.00
Step 2: Distance optical center = 10 mm from reading level.
Step 3: Prentice's Rule: = F x dec (mm) / 10
R: = � 2.25 x 1.0 = 2.25 base down
L: = � 5.00 x 1.0 = 5.00 base down
Step 4: Difference: The left lens contains 2.75 more base down prism than does the right eye.
Step 5: Slab off the left eye 2.75 base up for conventional slab off or slab off right eye 2.75 base down for reverse slab.


Exercises: Calculate the amount of conventional and reverse slab off needed to neutralize the vertical prismatic imbalance in the following prescriptions. Assume a 10mm reading level.

1.) Rx: + 1.00 � 0.25 x 90
+ 5.50 � 0.50 x 90
Add: +3.00
2.) Rx: + 4.50 � 0.50 x 180
+ 0.25 � 0.25 x 180
Add: + 1.50
3.) Rx: � 0.50 sph
� 4.25 + 0.75 x 90
Add: + 2.25
4.) Rx: � 9.25 � 3.00 x 180
� 5.75 � 0.25 x 180
Add: + 2.75

< Page 17 of 19 >

1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19