# Optical Forums > Ophthalmic Optics >  Slab-off Prism

## Neophyte Optician

Is slab-off only used in presbyopes, or does vertical imbalance affect single-vision users as well?

Thanks!

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## chip anderson

Effects both but SV users can duck thier head and see through center of lenses at near and prism imballance is not a problem for most of them if they have normal movement of head and neck.

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## Neophyte Optician

Does this affect both FT wearers as well as progressive wearers?

Thanks!

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## TPL

Slab-off can ONLY be ordered with multifocals( slab is in the lens of least plus most minus or now in plastic; reverse slab is popular; consult with your lab before ordering) if a single vision patient experiances diplopia when looking down
because of a prescription differance have them look down as to look directly thru the optical centers eliminating the imbalance.Sab-off is desighned to correct verticle imbalance created when reading with bifocals.

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## chip anderson

Hate to disagree with a learned collegue but you can get SV slab-offs, any *good* lab can do this in house.  Have dispensed same before, but as I said most SV patients just tilt the head down to correct maintain vision though lens center where prism is not a problem.

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## HarryChiling

> Does this affect both FT wearers as well as progressive wearers?


Yes this does effect progressive wearers.  The problem lies in the amount of prism when looking away from optical center.  If the power is significantly greater in one eye than that eye will have greater prism the farther you move away from the optical centers of the lens.  The slab-off bifocal is done to equalize the prism imbalance.

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## CME4SPECS

> Slab-off can ONLY be ordered with multifocals( slab is in the lens of least plus most minus or now in plastic; reverse slab is popular; consult with your lab before ordering) if a single vision patient experiances diplopia when looking down
> because of a prescription differance have them look down as to look directly thru the optical centers eliminating the imbalance.Sab-off is desighned to correct verticle imbalance created when reading with bifocals.


Sorry, but you are incorrect. Single vision lenses can have imbalance. 
Air-O-Lite even makes a semi finished SV with a reverse slab.

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## TPL

I was hoping that the optiboard members who have used slab-off on single vision lens wearing patients could post any sources that they know of thatwould explain the formula to determine as how and when to do this.I was taught in college(Erie Community,Buffalo,NY) and have had New York State licensing exam questions in how to  solve slab-off questions in regards to multifocal patients but never a single vision scenerio and would like to become educated on how to properly determine as to when and how much to use in single vision  as I do with the mathmatic formula to use multifocals.I went on a web search for slab_off and all the information that I can find only uses multifocals so what referances you can supply me would be apreciated.

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## CDOT

> I was hoping that the optiboard members who have used slab-off on single vision lens wearing patients could post any sources that they know of thatwould explain the formula to determine as how and when to do this.I was taught in college(Erie Community,Buffalo,NY) and have had New York State licensing exam questions in how to solve slab-off questions in regards to multifocal patients but never a single vision scenerio and would like to become educated on how to properly determine as to when and how much to use in single vision as I do with the mathmatic formula to use multifocals.I went on a web search for slab_off and all the information that I can find only uses multifocals so what referances you can supply me would be apreciated.


Sorry no references but you are over thinking it...take an average of 10mm drop on a sv, ft or progressive on a +300/-300 its the same imbalance it will affect some more than others.:)

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## Dave Nelson

Although single vision bicentrics are doable, I can't think of any reason whatsoever to actually use them. Even if the client insists on dropping the eyes to the lower part of the readers, (giving themselves the imbalance) the solution would be to drop the ocs about 6 or 8mm. Bingo: no imbalance. If you did, however, wish to calculate the slab, you would do it the very same way as with bifocals. Calculate the powers on the 90 degree meridians, use prentices rule to calculate the amount of imbalance, then slab it. Unlike bifocals, however, you would have to specify the hight of the slab line.

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## braheem24

> Although single vision bicentrics are doable, I can't think of any reason whatsoever to actually use them. Even if the client insists on dropping the eyes to the lower part of the readers, (giving themselves the imbalance) the solution would be to drop the ocs about 6 or 8mm. Bingo: no imbalance. If you did, however, wish to calculate the slab, you would do it the very same way as with bifocals. Calculate the powers on the 90 degree meridians, use prentices rule to calculate the amount of imbalance, then slab it. Unlike bifocals, however, you would have to specify the hight of the slab line.


You would then create an imbalance at the distance, best way around the slab is to fit the patient with a shallow "B" frame that forces them not to deviate too far from the OC.

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## Darryl Meister

You could also determine the reading level for a single vision slab-off by having the patient fixate a line of text, after assuming his or her natural reading posture, while you lower a card in front of the demo lenses of the frame. Once the card occludes the patient's vision, you can note the vertical position on the demo lens.

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