Can anyone see any point for slabbing a progressive? If so please tell me why?
Can anyone see any point for slabbing a progressive? If so please tell me why?
Jim,
I just today got a call from one of our users asking how to slab a progressive. My first answer
was about the same as yours..."Why?". Since his customer ordered one he needed a better answer. My first question: Where do you put the slab line? Why do you put it there? At what point in the lens does the imbalance become great enough to slab off? Are the rules the same for all progressives? Long and short corridor? We decided to have him ask the customer these questions. My personal belief is that it probably shouldn't be done. What say the brains on this Optiboard?
Wes Trayner
The reason for slaboff on a progressive is the same for any other multifocal--vertical prism induced by the difference in power between the two lenses when the wearer gazes downward to read.
I agree, Wes, that if someone needs a slaboff progressives are not a very good choice. Why go to the expense, trouble, and adabtation problems of a no-line lens only to stick a line on one side.
If all else fails and you just have to do it, would't you place the slab line approximately where a trifocal would be placed in a lined bifocal?
Why would you or should you slab a PAL? ..for the same reasons you slab at anytime, because of the power and the vertical imablance due to the distance from the distant OC related to the OC in the reading area...
Now where to slab a PAL? .. This is the tricky part, technically it is usually done about mid way through the intermeidiate, but I have ended up in some cases down nearer the reading point and in some higher up near the 180.. you should have a really great understanding of slabbing and it is useful to have a test set of prism on hand to double check... the higher the imbalance the higher the slab line in a PAL is a good rule of thumb...
If you do NOT place the slab properly and just "grind it on the 180" chances of non adapt do increase like crazy.. mixing cooridor lengthes (by design) is also a no-no.. they still have the same convergence spot OU it is the "power" that is causing the shift of image in the vertical merridian NOT the convergence point..
Also grinding prism thinning will NOT help but usually hurt since you are moving the ground OC upwards instead of down where it is at do to the design.. by moving it upwards you actually increse the amount of splitting.. if they have a few degree imbalance and they decide not to slab I will tinker with the prism thinning and instead of inducing 2/3's of the addition I will only grind around a flat .50 or .75...
I think that is the portion where a lot of people get confused is thinking it is a convergence problem more so than an induced power problem related to the OC's in the reading area..
Jeff "anyone want to fill in the blanks I missed" Trail
Nice answer Jeff. I was just going to log-on and ask Jim "why he thought you shouldn't?" but you explained it well. I must add though, that like the discussion Wes stated, there seems to be some confusion on where the line should be placed. And since there is confusion, it seems easier to answer "you don't slab a progressive". I have personally made slabs from @ the OC point, to 10 mm's below the OC point. So, as you stated, it varies by the need of the patient.
J. R. Smith
I would think that a slab on a progressive would render the intremediate part of the progressive useless, a disservice to the patient. Sure, you get the money for the progressive, and a slab. Why not just make a flat top or some other bifocal?
Why do you slab a progressive? The answer from a wholesale lab is, "that is what the account wants".
We have been slabbing progressives for about 7 years, plastic and glass, and found that there is the same do over rate as on a non-slabbed progressive.
It is always helpful if the account specifies how high the line should be. If they don't we slab the lense at the 180 line.
Right about this not being a convergence problem (a 180 degree issue) because the prismatic effect, hence the image displacement (at 90 degrees) is truly 90 degrees apart.
As far as the prism thinning on progressives goes, as long as the same amount of prism is ground on both lenses, there should be no effect on the prismatic vertical imbalance that already exists.
The other issue to take into account is the frame dimensions, actually the "b" measurement.
With todays smaller frames, short corridor lenses and fitting heights of 15-16 mm there is the possibility that slaboff might not be needed in small frame due to the shorter "reading depth." Rember that the amount of induced prism increases the further the eyes "travel" down the lens.
Jim Stone said:
I would think that a slab on a progressive would render the intremediate part of the progressive useless, a disservice to the patient. Sure, you get the money for the progressive, and a slab. Why not just make a flat top or some other bifocal?
Jim,
Missing the big picture here, the intermediate loss is nothing compared to have an induced doubling and trying to focus to read :-)
The still do feel a discomfort in the intermediate but not as much since you have a longer focal length and less induced imbalance..one other thing to consider (on the retail side) is that probably 90% of those emerging prebyops do not really need a intermediate to begin with ...oh it does help comfort wise but can they function, yes..I think it is a generational thing, where now it has become "habit" as soon as anyone has emerged the fist thing we all start talking about is "no-line" or "invisable" BI-focals..which it really a TRI-focal :)
I have a lot of MD's as accounts being in FL. I do get my fair share of cataracts and do have slabs often..but it is a job by job basis that has a lot a factors to consider.. are they wearing a PAL presently? How much of an imbalance has been induced, and how long or does the other eye need or have cat's removed? ..If so I than can give them a clearer answer if slabbing or even frosting or occluding may be the better option, even at times just placing a plano lens in the frame and waiting that few weeks or month before the other procedure...
I guess what I am saying is that it really has no set rule to go by and each time a lot of things need to be considered
What it amounts to is you have an induced problem and you need to meet the needs of the patient, they are depending on YOU to explain it and give them the thing the need and want to correct that problem...
BTW I have moved PAL's back into FT's for the slabbing as well, just for comfort and ease of use and even for economic reasons ..it's a case by case basis.. some answer huh? :)
Jeff "to slab or not to slab..that is the question" Trail
In 13 years of dispensing, I believe I fit 5 PALs with slab-off. My personal record was 2-2-1, so I suppose one could say it was truly a 50-50 proposition for me.
As Jeff pointed out, one really has to understand what slab really does. Technically, the slab is only going to resolve vertical imbalance at one- and only one- point below the optical center of the lens. So, if you place the line at the 180 with enough slab to resolve diplopia at the reading verification point, you may actually induce diplopia farther up in the lens!
I would suggest that the dispensers try PAL scripts without slab first. IF the patient has trouble with the eyewear, you can then use a set of Fresnel prisms of various powers placed at various points in the lens to determine the strength and positioning of the slab. This is how we managed to have success with the two patients who were happy with their slabbed PALs.
Trying to fit slabbed PALs "cold" is nearly impossible.
Another trick I always wanted to try- but was never quite brave enough- was this. Let's say you have the following Rx:
OD -1.00 sph
OS -4.00 sph /+1.50 OU
Try putting 0.75 BD prism in the OD lens only. This will create some vertical imbalance in the distance, but will partially relieve the vertical imbalance through some of the corridor. Just a thought, like I said, I never tried it. It would theoretically be the same as placing the optical center on the seg with a similar Rx used in a bifocal.
Good luck!
Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74
Tsk tsk..Pete, you are forgetting a few things (but by tinkering is how we learn even on the theoretical plane):) ... here is where you would run into some problems..in design, a semi aspherical design has already induced problems moving from the fitting cross and you would actually be making it worse by using your "theory" ...even tough we would by design have no induced vertical imbalnce a certian point in the lens as you move from that point the imbalnce increases dramatically do to the power..in the -1 (OD side) you have more physical surface where the abberation can carry without discomfort while in your -4 (OS) the distortion is increased by the physical property of the lens (semi-aspherical) multiplied by the power (minification in this case) and since we work on a monocualr basis you have a greater degree of discomfort..Pete Hanlin said:
Another trick I always wanted to try- but was never quite brave enough- was this. Let's say you have the following Rx:
OD -1.00 sph
OS -4.00 sph /+1.50 OU
Try putting 0.75 BD prism in the OD lens only. This will create some vertical imbalance in the distance, but will partially relieve the vertical imbalance through some of the corridor. Just a thought, like I said, I never tried it. It would theoretically be the same as placing the optical center on the seg with a similar Rx used in a bifocal.
Good luck!
The big difference between using your theory on a FT, which by the way I have done many times, pushing the vertical induced imbalance do to the near OC, down past the convergence point.. so technically we are running the same OC distant be it lower than normal and inducing power imbalance in the distant as well we are spreading the acuity.. so instead of say 20/20 we end up with a BCA of 20/25 .. you still need to do it OU.. in a PAL you are fighting it on multiple levels and you have forgotten to consider the actual design surface and how that will have an effect as well..If you had a true aspherical surface design it may work better.. since we have a connected pathway and work monocular instead of binocular it won't really help doing it to one lens only in a PAL..
Good try but in this case I think you're trying to make orange juice out of apples :)
Jeff "isn't optical theory a blast" Trail
Over the years I have fit several dozen pals with slab-off, all have been pal users, prior to needing a slab-off. All have been successful in wearing them. All slabs have been at the prism reference point.
The first time I ordered one from the lab, they said you don't do slabs on pals, because it defeats the purpose of no line. That made no sense to me, so what do you tell the patient? You can't have a pal any longer, because 1 lens will have a line and a little image jump at the line. Do you tell them they have to have a trifocal now and one lens will have 3 lines and the other will have 2? Or do you tell them that they have to have a bifocal, which means they no longer have the intermeiate that they have been accustomed to? So, I just had the lab make that first one 15 years or so ago, and never looked back...they work just fine.
On another note: I have a 20 year patient that wears
OD -6.00 and OS -1.50 single vision, he can't wear single vision with out a slab! We tried once and he couldn't tolerate it.
Terry Agin
Jim,Jim Stone said:
I would think that a slab on a progressive would render the intremediate part of the progressive useless, a disservice to the patient. Sure, you get the money for the progressive, and a slab. Why not just make a flat top or some other bifocal?
The intermediate on a pal is, at best, marginally functional from the get go. A slab-off won't make it worse or better. There will be VI in the intermediate regardless of the slabs position, unless you ignore VI at the near point. I would position it like a lined seg set on the low side. But I wouldn't slab a progressive. If the frequency of near task is below average, most folks will tolerate 3,4, even 5 diopters of imbalance without loss of fusion or notable discomfort. If near tasks are average to above average I would use a separate pair(s) of glasses, with the pals kept for general use. I can think of numerous scenerios and dozens of solutions, which leads to...
Jeff's Statement:
"I guess what I am saying is that it really has no set rule to go by and each time a lot of things need to be considered."
May be the understatement of the year.
Robert
Talk about a telling statement..sheesh..this lab guy should be SPANKED!! :) ...confusing cosmetics for visual correction is a NO-NO, which more or less is what they were talking about.CME4SPECS said:
The first time I ordered one from the lab, they said you don't do slabs on pals, because it defeats the purpose of no line. That made no sense to me
Sounded good if you say it REAL FAST I guess, but you were correct to think "this makes no sense to me" :) .. They used that as an excuse but I would tend to link the problem more towards it being a nightmare slabbing a PAL compared to a FT where we can use the seg line to help guide us with maintaining the slab line on the correct axis plane..that with doing a slab is more labor intensive and having all that work go to waste because of "non-adapt" is frustrating on the lab guys end..
You can slab PAL's.. and it can work, but having both ends (lab/optician) knowing what they are doing and being able to communicate sure improves the results
I think that is one of our major problems we have, we have confused add ons with actual visual correction.. the highest retail sale has taken the place of the best visual answer in a lot of cases..
I get orders constantly for 1.66 and 1.67 and 1.71 SV and people who are -2's and such...come on.. I have one OD who distributes .75 adds in PAL's like it was the flavor of the week, has even ordered .50 adds!!!! .. It is great to make a bunch of money, but the most expensive way is NOT always the best way :)I do it, since it is written on my invoice and ordered, but do I think a lot of it is ethical? ..nope.. I just feel sorry for that guy who walked in and walked out with that .25 change in distant RX or NO change and now needs that .75 add to function..oh and just had to have a AR coat...
Sometimes I feel sorry for you guys on the retail side, it seems ethics in optics has become a four letter word in a lot of places..
You guys that are still being ethical I bow to you and give you a high five..I figure it must be tough to not sell that sucker who comes in wearing a -1.5 SV and the last place they went to sold them a 1.67 with with every add on under the sun..My biggest frustration when I owned my two stores was when I explained to them it was overkill and this option would be less expensive, they figured I didn't know what I was talking about
Jeff"OH MY a pl.-.25..you HAVE to get 1.71" Trail
Thank all of you for your responding. This was my first post and I have learned a lot from your answers. I have to say that I stopped considering progressives for cosmetic reasons a long time ago. The porpose that I felt was defeated by slabing was loss of vision in the intermediate zones of the lenses. As stated above, you can only match the vertical prism in two spots on the lens. Once you start moving away from these spots you are right back in an imblance situation. Often these patients are in between cataract operations. I suggest trying without the slab first. There is a heck of a lot of people reading sucessfully with one eye, many by design (mono fit contacts and believe it or not lasik monos). I relize I am addressing the most educated of the optical industry, but you have to realize for every educated one there are fifteen untrained newcomers that depend on the labs to advise them. Thank you all again for your thoughfull replies.
I suggest trying without the slab first. There is a heck of a lot of people reading sucessfully with one eye, many by design (mono fit contacts and believe it or not lasik monos).
Jim,
I believe you have hit on the main thing- namely, go with what works for the patient!
Sometimes we get so caught up in "trying a new fitting trick or the latest product" that we forget that all the patient really desires is functional, comfortable vision.
Robert,
I've never worn PALs, but most of the patients I've fit over the years seem to get a lot of use out of the intermediate portion of the lens. I think the quality of the intermediate (and the periphery) is what truly makes or breaks a PAL. If all we are hoping to do for our patients is go from distance vision to near without a line, we should all save a lot of money and effort and just fit blendeds!
Jeff,
I usually follow your line of thinking pretty well, but I have to admit I'm unconvinced about your arguments against using prism in the entire PAL to "split" vertical imbalance between the lenses. Perhaps you can elaborate sometime over a few Coors and I'll see the "light" (pun intended).
Pete Hanlin, ABOM
Vice President Professional Services
Essilor of America
http://linkedin.com/in/pete-hanlin-72a3a74
Pete,
Either you are over estimating or I am under estimating the usefulness of the progressive zone of PALs. The truth probably lies somewhere in between. :)
Robert
If you think the progressive zone is useless, you are probably using Variluxes are are just too young to appreciate this usefull part of these lenses. As you get older, in bifocals, a fuzzy spot appears at about 16 inches and widens to six feet. As you get older. Correctly fit progressive lenses solve this problem. Progressive lenses are not just for cosmetic purposes. :hammer:
Jim,
Well, I didn't say useless, I said "marginally functional". I was thinking useless though.:)
I have been wearing them for ten years. Rx is -4.50 with a +2.00 add. I can see the speedometer, items at the far end of my desk, etc. I can not wear them comfortably when working with my clients, or looking at the monitor. My point is that if you have tasks to perform at 20" to 30" standard progressives (full distance power and full add power) are not the best choice.
Robert
Robert
You are so right. (By the way I was only kidding about Variluxes.) I have a pair of Rodenstock Office for the computer. They keep from constantly rocking my head back 15 degrees to clear up the screen. The Sola Access is also good for that. There are more lenses for that I am sure. Thanks for your comments.
Jim
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