Slab Off and Slab On????

[mizzrai]

Am pretty new into the Optics world. Am confused completely about slab offs. And as well an bifocals, can you slab off varifocals.??.and how is this done?? Am i right in saying you slab off the most negative and slab on the most positive?? And I assume this is usually done on anisometropic prescriptions....Can someone please help and explain these terms clearly.(giving an example if possible). This is done to reduce the prismatic effect at near??
Anyone now of any good sites that can help...??

[Happylady]

My sister in law has a slab off on her Varilux lenses. Her one eye is a +3.00 and her other eye is a -.50. She does well with it. There is just a slight very fine line across the middle of the lens.

[lensgrinder]

And as well an bifocals, can you slab off varifocals.??.and how is this done?? Am i right in saying you slab off the most negative and slab on the most positive?? And I assume this is usually done on anisometropic prescriptions....Can someone please help and explain these terms clearly.(giving an example if possible). This is done to reduce the prismatic effect at near??

Greetings mizzrai. I assume when you say varifocals you are talking about progressives? Anyway yes they can be done on progressives and you put the slab, not on the most negative, but the most minus least plus. It is not necassarily done on anisometropic rx's either. If someone has 1.5 D of prism or more at the reading level then you would consider doing a slab-off or reverse slab-off. To figure prism at the reading level use prentice's rule P=c X f. As a general rule use 10mm or 1cm to figure prism at the reading level
P=Prism
c=amount away from o.c. in cm
f=lens power
Example:

O.D. -1.00 -3.00 X 180
O.S. 0.00 -1.00 X 180
Add +2.50
Reading level 10mm

First you figure power in the 90th meridian
O.D. -4.00 @ 90
O.S. -1.00 @ 90
Second you figure prism in each eye
P=1 X 4.00 P=1 X 1.00
P=4 Diopters BD of prism P=1 Diopter BD of prism

Third since the prism is in the same direction you take the difference between them.
P=4-1
P=3 Diopters BD prism in the right eye
Since the right eye is the most minus you would slab the right eye with 3 Diopters of BU prism.

Hope this helps

[mizzrai]

Thanks, understand it alot more than before