# Optical Forums > Ophthalmic Optics >  Slab off Vrs. Reverse slab off: when,why?

## Alteaon

I do apologize if this has been discussed, but I'm still slightly confused.
I am aware of slab off and reverse slab off, but why would you want to use one over the other? 

What kind of situation would lend itself to reverse slab off rather than a "regular" slab off?


Maybe I'm just not thinking straight...
Thank you for humoring me

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## tmorse

> I do apologize if this has been discussed, but I'm still slightly confused.
> I am aware of slab off and reverse slab off, but why would you want to use one over the other? 
> 
> What kind of situation would lend itself to reverse slab off rather than a "regular" slab off?


Regular Slab-off is ground Base Up by the lab to solve the Vertical Prismatic Imbalance (VI) at near vision, due to anisometropia or antimetropia (a power difference between OD and OS at 090 degree meridian). Difficult to do by some lab techs since it's so rare. 

Reverse Slab-off was developed by Vision-Ease and has the slab-off line
pre-ground Base Down at the seg line, so it is placed on the opposite eye to regular slab-off. (remember you slab-off only one eye) 
Labs only cut the back curve for distance which thay can do rather well. No mistakes in slab-off line position this way.:cheers:

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## Darryl Meister

> Reverse Slab-off was developed by Vision-Ease


Are you sure it wasn't Aire-O-Lite?

We should add that reverse slab-offs are thinner in Plus powers and generally have more consistent quality (including the quality of the slab line), while regular slab-offs are available in more lens designs and materials since they can be applied to most semi-finished lenses. The end optical effect is the same though.

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## Fezz

Both types of slabs will give you the same effect. The choice of which one comes down to a few basics. What lens material do you want? What seg style? Reverese slab is pre made(molded in the blank) in limited materials and seg styles. Regular slab can be done on just about every type of seg and lens material. Hope this helps.

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## Fezz

Below is a few links to a articles on slab-off.(Thanks to Harry Chilling's optical search)


www.nfos.org/degree/opt12/module_09c.html


www.opticourier.com/1webmagazine/2006/01jan/content/lens_corner/index.asp written by a fellow Optiboarder, OptiDonn.


You can try the Younger Optical website. They have a FAQ section about slabs. I had trouble posting the link.

Hope these help

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## Brian L. Schnirel

Hello,

Although the above statement is basically true, minutely the reverse slab would produce better optics for the patient in theory. This is based on the Reverse slab being closer to the natural point of correction by virtue of reduced vertex distance as opposed to the positioning of the Obverse (regular) slab.

Sincerely,
Brian

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## tmorse

> Are you sure it wasn't Aire-O-Lite?


No Darryl... not sure at all. 

The Canadian lab that ordered one of the first reverse slal-offs for me said they got it from Vision-Ease in San Francisco. This was back in he middle 80's and you couldn't get this lens in Canada.
:cheers:

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## Barry Santini

which lens has the most/least prospect for the Rx changing, and then use the appropriate slab off on the lens least likely to change

My two cents, what's yours?

Barry

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## EyeFitWell

Is a slab-off expensive?
I don't think I've ever seen an invoice for one...

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## Fezz

> Is a slab-off expensive?
> I don't think I've ever seen an invoice for one...


 
It depends which lab:

Lab#1=$75
Lab#2=$60 standard. $65 reverse
Lab#3=$70
Lab#4=$45, reverse$40

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## Fezz

Just for fun, here is a lab website that calculates slab for you. 

www.robertsonoptical.com/slaboff.cgi

Our own Darryl Meister also has a vertical prism calculation tool on his website: www.opticampus.com

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## Michael Walach

Reversed Slab-off is actually Sab-on; you add a slab to the lens.

Slab-on can be made on the back of the lens the same way as Slab-off using DAC International RxD Direct Surfacing (Free-Form) technology. This can be done on any lens type or any lens material.

Michael Walach
QUEST OPTICAL SPECIALTY LAB
questlab@tampabay.rr.com

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