What Is A Slab-Off and How Do We Calculate It?

Slab-off is a prism technique used to correct vertical imbalance. We’ll try to first understand the concept and why it is needed then work through an actual calculation. Slab-off is most often used on lined multifocal lenses but can be used on progressive and even single vision lenses.
Learn More: opticianworks.com/
This week I do my best to help you visualize the concept of slab-off. Slab-off is a prism technique used to correct vertical imbalance. Opticians will request slab off when the patient has anisometropia. We will work through the how and then do an actual calculation. It is through optician training that opticians learn the steps involved in solving for slab-off calculations. Slab-off is most often used on lined multifocal lenses but can be used on progressive and even single vision lenses also. Opticians using slab off calculations may need to determine the power in oblique meridians and apply the concept of the optical cross in their work.
Learn More: opticianworks.com/
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This video lesson is sponsored by Laramy-K Optical
Laramy-K Optical is a digital uncut and coating lab unlike any other. The only exclusively uncut lab in the country; we provide our customers with independent, high-end alternatives to corporate brands. Through craftsmanship, technology, and independent, innovative vendors like Younger, IOT, A&R, and Quantum Innovations we offer our customers the Integrity portfolio of freeform lenses and coatings. Made in the USA, the Integrity brand is not a generic house-brand, but is the absolute best available, designed to give the wearer that “wow!” experience.
In addition to the digital line, we still offer conventional surfacing, including glass. Maintaining traditional equipment gives our customers vastly more flexibility in their dispensing, particularly when it comes to more difficult prescriptions.
Learn More: www.laramyk.com

[@luisgomez1211]

I'm an optician for many years, maybe 30, but I never end to learn something about optical, or remind me...I like your videos...

[@tarapishgharavol5986]

The way you explain complex concepts is one of the best I’ve ever seen

[@LaramyKOptical]

Thanks.

[@optimistic_1115]

I never really understood this as an optometry student but now I finally do! This is very helpful. Thank you!

[@LaramyKOptical]

I'd rank it up there pretty high on the confusing concepts list. John

[@Rex-ou3dk]

John, I had surgical peal on the retina of my right eye several months ago. The eye is now stabilized enough for new glasses. The optometrist was crisp and clear. “You have an issue with the retina and slab off corrects it.” My work with the Amsler Grid tells me that. Your video actually explains what is really happening. From a patient, thank you, Rex

[@sophiamoh292]

This was such a helpful explanation! Thank you so much!!

[@skibickiadrian]

Thank You so much! Have a nice day.

[@lisaboyd8493]

Thank you so much for your informative videos..They helped me pass my ABO exam!

[@LaramyKOptical]

You are welcome, congratulations on your ABO! Please help spread the word about OpticianWorks.

[@user-dm3qn2zp7g]

Tomorrow is annual exam of spectacle lenses and contact lens ,this video might be helpful 👍👍👍👍

[@ShaniAvraham]

So helpful! Thank you :D

[@TheBallsinyoumouth]

I've sold slab off twice in my life and the first time I recommended slab off to a patient, his rx was a OD -4.00, can't remember his cylinder but his other eye was Plano. As soon as I saw this, I spoke to him about reading and he said he had trouble with reading and that's when I recommended it. Second time, I recommended it to a patient with OD-7.00 and OS- 2.00. I can't recall the add powers but I remember recommending it for them. I even went as far as to clock their base curves. And I've only been a licensed optician for 5 to 6 years. Bicentric grinding is my favorite topic on the ABO and I can do these problems all day long.

[@TheBallsinyoumouth]

Most of my message was cut off. Not sure why. But I was saying I recommended slab off to a -7.00 and - 2.00 and the other person was - 4.00 and Plano

[@LaramyKOptical]

Well, I think we can be 100% sure that this will be the first and last time we ever see, "...my favorite topic." and slab-off in the same paragraph. John

[@amandaurnberg6087]

Thanks Laramy so helpful

[@NoneMechanic101]

Thank you

[@shawnscherbroeck1197]

Isn't conventional slab off ground base up on the lens with the least plus power? Therefore, would be 6 BU Right eye?

[@miserimuslovestarvus6587]

Very clear explanation and presentation. Thanks. So, as a shortcut, can we say in bifocal lenses, added prismatic effect equals the power at 90°?

[@LaramyKOptical]

Miserimus, No, it just happened to work out that way because of the powers used. Sorry, I know it is a little confusing and if I do this again I'll use different powers. Thanks John

[@user-bn4lz3kq3i]

How can I calculate the rate of magnification changing related to the changing of distance from my eye to my PC screen?

[@cpt4168]

Hi John, I'm confused on the end of this video... you say add the cancelled prism (6 BU) to the "most minus, least plus" lens... wouldn't that be the right lens? On the written out part on the bottom of your equation, it says "6prism BU L "

[@LaramyKOptical]

c Pt, That is the total prism error created. You must figure that out first and only THEN follow the rules and correct the imbalance. If you listen to the video it is correct. With the benefit of hindsight I could have done things a little different but... John

[@blairtaylor9552]

You only touched on the fact that this is 'mostly' used on bifocals, but also on progressive, and can also be used on single vision. Would sure like to see a part 2, where you explain the most common applications of this, where it will do the most good, etc.; and the basic differences between using it for each type of lens? Please! :-) … I'm an optician, and have to troubleshoot regularly. The other day, a patient came in for refraction: he was 'approx.' -3.00 in the OS, and the doc prescribed 'approx.' -8.50 for the OD, which he honestly had little vision using his current specs.. Common sense and past patients' tells me that it's too big a jump in the first place. In the second place, there's a HUGE difference between being in the chair... motionless/in a darkened room/with small perfect aspheric lenses/with perfect AR; than what he will be trying on later in his eyewear.... loads of scattered light everywhere/large frames/spherical aberration/possibly spherical lenses of lesser quality/and different AR coatings. Many times I've seen them have to back off to a midpoint of power somewhere, to have eyewear that is at all comfortable enough to be useable for any period of time. I have to follow the doctors' lead of course, as they are trying to find if a refraction is actually possible for sharp vision in the OD eye. We will have to wait and try the first Rx, and then possibly make a change, but when we come to the final 'comfortable/useable' power for the patient in that strong eye, I'm wondering if the last step is to order the same Rx now but with a slab-off in the OD, even in these distance single visions, to correct for nearer focus in that OD? So much to learn! It really never ends! Thanks much for all the videos- I know much of it all, but ALWAYS pick up new knowledge here and there from your videos and your website. You REALLY need a person or two helping to compile your direct and common sense teaching style into 'THE' comprehensive opticianry handbook! What I've found so far is really scattered among so many courses and books and CEC modules, and many are very very VERY poorly written. Sure would like ONE really good comprehensive book on the subject with all this in one thick volume! Tks

[@LaramyKOptical]

Um, let's see... 1) Slab-off is a 95% lined multifocal, 5% progressives and 0% SV. The imbalance corrected by slab-off only occurs when you have an add power. 2) Extreme differences in power between two eyes is fairly common and has several different names. Yep, the best possible frame fit, best possible measurements (OptiKamPad) and the best in lens design will be your best option. 3) I did write a complete textbook presented in a logical order and covering what an optician needs to know TODAY it is called OpticianWorks.com!

[@pankajgarg7737]

What is practical examples of slab off

[@Mel-yi5gi]

I love watching your videos. It is helping me a little. I am 3 months in optician world 😁. I have not watched all your videos yet. I am not sure if you have done a video of Nikon lenses. It is a little confusing. I would like to get a better understanding.

[@LaramyKOptical]

OpticianWorks.com is the place for you - literally!

[@Mel-yi5gi]

@@LaramyKOptical Thank you! 👍

[@LaramyKOptical]

Sorry - I missed the Nikon mention. We are a wholesale optical lab that produces our own lens series based on IOT designs, we don't handle the Nikon brand. Last I knew in the US it was still a bit of a niche brand with somewhat limited availability but maybe that has changed? In many ways (not all however) a lens is a lens is a lens these days.

[@christopherjulianus8472]

It’s a long story but I used to be an Option and today I got my new glasses. OMG!!! They didn’t even get the prescription right!

[@hell00O0]

Hi John, Thanks for the slab-off video: I understand that to compensate, we grind same base prism in most minus or least plus of the two lenses. When we say that slab off will be ground for the base direction deteremined by above calcuation in the lense that is most minus or least plus. In a Rx that has R: -8.75 and L: +4.25. Does it mean that slab off will be groun in right eye as it is the most minus or least plus? The answer to this problem is 2.70 prism diopters. It is the problem from your forensic opticianary video where a guy's insurance coverage was denied and you proved that he was seeing through prism of 2.7 pd. Thanks again, looking for your help in understanding the slab off on this one.

[@LaramyKOptical]

I'm not 100% sure how to answer this. That example is about vertical imbalance and that is not corrected by slab off. That would be corrected by remaking the lenses with the correct OC placement (same in both eyes). If you took those powers and needed slab off to correct for image placement then you would slab off the -8.75 which meets the most minus least plus idea. It isn't if the plus is weaker it is a case of if you have plus/plus you slab off the weaker plus. Regardless as I mention in the video, don't make yourself crazy over it. Let the lab figure it out, it is kind of their job and these days they just plug it into the software and it tells them/you the best options. It is good to understand the concept of slab off but spending hours learning about it, memorizing it and working problems about it is, in my opinion, a waste of your time. Spend it instead on understanding compensated lens design, something you will see everyday and may well get questions about from actual real live customers every day. It will serve you well in the actual day-to-day working life of an optician where slab off will not. John

[@ItsPainnz]

@@LaramyKOptical Ok but so you dont need a plas blank to do slb off? if you just can mold it or grind it in it should be easy! my helpline said we dont offer slab off but if the patient has induced vert imbalance but no induced prism shouldnt they be able to do it?

[@LaramyKOptical]

@@ItsPainnz I'm not 100% sure what you are asking? If a lab can't or won't handle slab-off I doubt you will convince them otherwise. Even with a premolded reverse they would still need to know how to do the grind. You can call Janet at the lab - you don't need to be a customer of ours - she would be in a better position to sort it out for you. 800-525-1274 afternoons CST.

[@AbhishekSingh-lu8tw]

Do we add Base Up to Minus near section too (of course we chose greater Minus)?

[@LaramyKOptical]

Always the same.

[@AbhishekSingh-lu8tw]

@@LaramyKOptical So, Base UP Prism is added to the near section of both types- either Myopic or Hypermetropic anisometropic presbyopes.

[@LaramyKOptical]

@@AbhishekSingh-lu8tw Gotta be 100% honest here - I'm not 100% sure! I don't have any examples of slab-off calculations where you have one eye minus and one eye plus. Seems to me that would kind of take care of itself? If you want to drop me an email with an example I can kick over to the lab and see what the software would tell us to do. Or you could do the same with the lab you use.

[@capncharlie7894]

I love this guy. He is the John Malkovich of optics.

[@LaramyKOptical]

Whoa! That makes two John Malkovich comments in one day. I wonder if he needs a stand-in understudy?

[@pankajgarg7737]

In this video what u do in last i can not understand clearly can u tell me in brief what u want to say about slab of

[@LaramyKOptical]

Slab off can be applied to a lens using free-form design. Just makes for a smoother blend and a little better optics.

[@sumeraarif1165]

Hi sir, kindly tell me which book is the best for dispensing spectacle ??

[@LaramyKOptical]

www.OpticianWorks.com

[@lynnettegarcia8125]

@@LaramyKOptical I would also add the ABO Ultimate Study Guide

[@amitverma3764]

Hi, question about reverse slab-off. Whats the lowest reverse-slab off that can be ordered. What increments can reverse-slab off is ordered. For Example, If the Vertical Imbalance is .60 D. Can I order .50 D reverse slab off, and leave .10 D residual (within tolerance) . OR Can I order exactly .60 D of reverse slab off. I searched the whole web, and cannot find the minimum reverse-slab off that can be ordered. As we all know that VI under .50 D is considered tolerable and can be ignored

[@LaramyKOptical]

Amit, We will get back to you soon. I'm checking. John

[@LaramyKOptical]

So, the lowest reverse slab that WAS available was 1.5. the highest was 3. They have been discontinued. We run conventional slabs (backside, most minus least plus) in house. This would have a definite visible line. The lowest is also 1.5, going up in .25 increments. The highest we've done is, if I recall correctly, is 12.00. The amount possible is dependent on the power, the size of the frame and the client's PD. We also run Freeform slabs, which stick to the same parameters, but the line is a bit blended, making it less noticable. The highest possible is 7.00 Hope this helps, Janet

[@Ahmed-jx2bp]

I'm reading an ophthalmology textbook and it says that a slab-off is "to remove the prism inferiorly from the more minus lens" ... Is it the same as adding a wedge like you did??

[@LaramyKOptical]

The rule for slab-off: -/- do the highest minus, +/+ do the lowest plus, +/- do the minus lens. I can't really say that you "remove" anything when you are actually adding something. I don't know what the word "inferiorly" means. Hope that helps! John

[@blairtaylor9552]

@@LaramyKOptical Superior(ly) usually referring to the upper; inferior(ly) to the lower. Since you mentioned it, I was going to ask anyway... the idea is that we're adding material in order to add prism, so why isn't it called 'SLAB-ON'??? :-)

[@LaramyKOptical]

@@blairtaylor9552 I believe it is a carry over from when a lens blank was created by hand. You would have to start with a blank thick enough to have the prism wedge and then slab-OFF the unwanted part above the line. I think.

[@notme1048]

As a patient all I could tell that for an extra $100 they put an extremely annoying line across your good (better) eye. THAT'S IT as far as I could SEE. I didn't even leave their parking lot it was so damn annoying. I went back and complained and NO ONE...NOT EVEN THE DOCTOR could explain: is the line normal, does non slab off NOT have the line, what does it do, how could this be better??? They ask me to just wear it for a few days but I couldn't get through half the day as it was like having your hand saluting in front of one eye. I took it back and said get rid of the line. 2 weeks later no line (assuming no slab-off) glasses are fine. BUT today i got the bill & I'm wondering why do i have to pay for them talking me into trying it? I visited here to better understand what slab-off is since my doctors didn't seem to know and just said it helps adjust the extreme difference in my good & bad eye Thx for letting me know what my doctors didn't. P.S. Now that i saw this video i can kinda see the double vision thing BUT my brain just does its best to compensate & i really don't notice.

[@LaramyKOptical]

Correct a slab-off will have a visible line, no way around it. Like everything else need creates acceptance. If you really needed a slab-off you would have learned to live with the line - in fact it does become "invisible" over time just as the line in an old fashioned lined bifocal does. But if the need isn't high enough it would be more of an annoyance than a help. Free-form surfacing can help reduce the intensity of the line but it is still there. Glad we were able to make things a little clearer for you. John

[@notme1048]

@@LaramyKOptical rrrrr...If they would've just explained as you just did I'd have saved some time & money. I did call the Dr. office and very politely pointed out their shortcomings and she seemed to agree and said she'd pass the information on to the staff...Hopefully it will help people down the road. Thanks so much for the quick response & good information.

[@quyonariley2604]

Confused on how you got 5 mm

[@ashe792]

I have myopia , my prescription for both my eye are the same S:-14.50D C:-1.00D AX:180

[@LaramyKOptical]

Miguel, Do you have a question? John

[@LaramyKOptical]

See also: kzbin.info/www/bejne/pWjPpoGwnpZmkJY

[@pankajgarg7737]

In last moment of video

[@williecook3178]

Your depiction puts the slab line on the OC, which is incorrect! The slab line gets ground on the top of the bifocal.

[@williecook3178]

33 year lab optician here.......

[@LaramyKOptical]

Good catch! If I ever redo this one someday I'll be sure to correct that. Thanks

[@heelswinagain]

Sorry, but I’m a little confused on why you took 2x10 and then divided it right back down by 10. I see what you did, but not why.

[@LaramyKOptical]

Marquise, Yeah, sorry about that. Things just worked out that way. I should have chosen some different powers! It still just boils down a pretty basic Prentice's Formula problem. Work through those videos and problems and then come back to this one and it will click. John