here's my scenario:man had surg to correct od for near&wears CL on os for dist-also wears progs over CL(mild dist rx)--he wants cheap back-up(single vision)pair for when he takes CL out-this rx is R+.50-1.00@90
L-6.00-1.00@100-with this he experiences double vision-i have never done slab-off w/SV..i'm thinking to order reverse slab-off for os & fit it @lower lid-is it worth it? remember he does have an add power-so even w/slab-off near still may not be satisfactory
Not so sure reverse slab off is appropriate. Its been so long, I forgot myself, but . . .
Try this basic rule of slabbing off least plus or most minus lens. My method of placing the line is to pass the narrow edge of a PD ruler DOWN from the top until it blocks the upper part of the victims I mean patients reading field of view!
Anything below that is the placement position of the slab off line. I tell my lab to place the line as though it was a seg height.
Since these are readers, the technique assures adequate window.
Slab off is used to correct double vision caused by an image that is separated by unequal prism effects. That effect is a function of looking down into the near segment and away from the optical center. (Prentice's Rule Dxd)
Slab off is not needed in single vision because the wearer can tilt his/her head down when reading and keep the gaze fixed through the oc (Thus making d=0 and killing the need for prism balance). I think what you have instead is double vision caused by unequal magnification (or in this case minification). The image in the right eye is only slightly smaller than actual, while the image in the left eye is minified greatly. This is a tough one! I've never done it myself, but I've read about using different base curves to offset this kind of imbalance. But, this rx is definately a challenge.
Good Luck!
cj
Remember that a slab~off only corrects image location. Patient may still experience diploplia due to image size differential.
I have had on occasion, to reduce or increase the power in one eye or the other to get image size the same. Remember that in a C~L this is seldom (but occasionally if the difference is enough) a problem, but In spectacles it can be a problem. Patient will also have to converge a little more in the eye with the most plus, and the slab~off is only for vertical displacement.
I agree with CJ. Slab-off is only used in multifocal lenses (since vertical displacement only occurs away from center, their is no need for slab-off in single vision lenses).
Good luck in attempting to resolve the aniseikonia with iseikonic lenses (might want to call your lab to ask them for advice). I believe what you will need to do is use the steepest base curve you can on the -6.00 lens and the flattest possible on the +0.50. Make the -6.00 lens a little thicker than normal (yeah, this should look really good, huh?). Both of these steps should decrease the minification effects of the minus prescription.
Good luck!
Pete
PS- Darryl???
I fail to understand term slab-off.Can any one of you please explain me in simple plain language what actually slab off is,when we use it? Sorry the question is very primitive.
Sara
Sara,
Read this message thread for an explanation of the term <FONT COLOR=#FF0000>slab-off</FONT>:
http://optiboard.com/ubb/Forum2/HTML/000657.html
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OptiBoard Administrator
It seems to me that there must be some need for slab-off in SV or they would not be available in SVSF reverse slabs. Anytime you get away from OC in greatly different powers there is going to be double vision regardless if there is a seg involved or not.
Nikvin, By the way, how much "add" power are we not using for these "without CL" glasses?, IMHO, if he wear an add over his CL, he needs it in these glasses. Pete is right,all the factors of magnification need to be considered, including base curves, lens thickness, index, vertex distance, etc. Since he really should have an add like his "over CL" glasses, he would need a slab-off. the glasses should be as small as possible, period. likely 4.00 base curves O.U., knife edge on rt. lens, likely 3.0 CT on left lens. prefer to AR coat, since ghost images can be a big problem with this type of set up.
First you do at times NEED to slab even a SV :) ..Second, Nikvin you are going to being slabbing the WRONG eye if you slab the OS in a reverse slab...MOST minus least plus is for a conventional slab. A reverse slab it's LEAST minus most plus :)If you are going to slab and use a reverse slab you need it in the OD NOT the OS.
Next.. magnification.. the amount of magnification in these powers comparing them on the "same" BC is almost a imbalance of 10% !! ...in the plus eye it's around 1% and in the minus side you have to deal with -8.75% ... by manipulating the base curves you can move the magnification or in this case reduce it but also using unlike base curves.
I would slab this guy even if it is a SV.. and it should work out just fine...
One other question though, is he doubling at all past "infinity" which we all know is based on 20 feet in an OD's office? One of the problems usually is that they tend to see great at 20 feet in the distant but start to double around 30 or 40 feet out to "real" infinity.
You have to figure into the end numbers a numbers of things that he is not dealing with now..vertex distance etc. that dealing in contacts you do not have to consider.
To do the math I figured with a vertex of 14 mm and corneal-pupilary distance of 3 mm ....
Another thing I strongly recomend it use a aspherical lens for periphral distortion. One other thing to consider is that if they can't fuse at a distance further then infinity is to possibly have some vertical displacement to help as well, since we know :) that in a minus to move an object you move the lens the opposite direction (against the motion) and in a plus it's just the opposite.
Well that's all I got ta say :) But then again I'm only a lab ratDarris got an opinion you or Chad'ster want to chuck into the pile?
Jeff "got my tickets for the Daytona 500 and can't wait" Trail
As for the availability of single vision slab off lenses, I can only think of two reasons...First, if someone is incapable of lowering their chin...as in serious neck problems...then he would be forced to look away from the oc at times and a slab off would help...The other reason would be strictly for profit. My lab charges an extra $50 for a slab off. If someone wants a single vision slab, why not sell it? :)
cj
Prism is prism is prism.
Anytime you move from the established location of the optical centers, and the powers of the lenses is not equal, you have the potential for vertical imbalance.
It is imperative that single vision prescription analysis be performed just as you would multifocal prescription analysis. Only after the analysis, using Prentice Rule and using the distance from the actual location of the optical centers to the position on the lens through which your patients views reading material, as the decentration component, can you know whether there is a vertical imbalance. Given the information provided, I am confident calculations will reveal vertical imbalance. However, I will concede that a better solution to the slab off is to position the optical centers at the point of gaze through the lenses if there is enough blank to afford a cut out. Using the "top down" method I described earlier, that would be a nice place to put the optical centers to minimize vertical imbalance.
Now, my Chamomile tea is getting cold.
Later, Dude!
CJ,
I don't know about you but trying to get a person to constantly just use a lens through the target zone is a dying cause.
Plus they would stay nauseous from all that doubling as the eye traveled around, (which is almost impossible to stop)
I seen a few posts that were started down the right road and had parts of the formula'.. but we have to remember that the higher the degree of power and imbalance of power the less the eye has to travel to start running into fusion problems, you also have to remember that the motion of the optics and images is playing a major part, remember the basic rules of power has an effect on the motion ? ... so even if they do want a SEG then you can still slab OR have to slab (in some cases).
I even seen a few of the posts talking about the basic reasoning behind slabbing and depending on the power AND the effect the OC in the seg has to come into play.. NOW look at a SV as a full field seg.. you would still have the same amount of induced imbalance because of the difference in power.. of course the amount would be less since we would start at an equal OC placement vertically, but as you moved in the visual field then you would have the SAME problems as you would have had to get a slab in a hard seg design..
So there are times when you should and can slab a SV.. and no not just for profit.
Going by the theory of "well since the OC's are equal and they can just always look through the OC and tilt their head" sounds good, but does it work always? Nope.
Do I slab a SV? I have over the years.. not very often and at times you can get by with manipulating the OC's (grinding them down OU. I have probably only slabbed a SV maybe a hand full of times over my years in optics, but I think I needed it all those times..
I think where most people tend to get confused is they mistake "prism" with "induced image imbalance by power" .. I know I had more then a few opticians ask me why grinding prism in a lens instead of slabbing the SV would not work.. Well, that more or less made the glasses unusable for the patient.. they do not have a fusion problem, at rest, but when they go in motion, so grinding prism through the total lens instead of slabbing makes the problem worse, not better.
I do not think it would be "greedy" at all to slab this lens, I think it is what's needed to best give the person the best corrected acuity.. in my book that is not greed but what we are supposedly trying to accomplish in our profession, right?
Oh well 'twas fun but I see I'm the loan ranger on this subject and we all have opinions and I do not want to step on toes.
So I'm done posting from my point of view maybe someone else has another solution they wish to toss out there.
Jeff "beans or optics don't know which I'm more full of" Trail :)
I tend to agree with CJ & Pete - the fundamental issue here is magnification.
The +0.50 -1.00 x 90 = +1.7% image increase
The -6.00 -1.00 x 100 = -8.0% image minimization
Net result is 9.6% difference - and the ideal is to stay within 2%.
The "fix" is not easy, nor is it pretty. The minus needs and/or a higher base and additional thickness to drop the %. Changes to the R_eye change little. The formula for calculating all this is found in most text books. I wrote an Excel worksheet on it because I got tired of doing all the hand work. I can post it here it there is any interest.
I'm afraid you're not going to sell me on the practicality of putting a slab in a single vision lens...
Just out of curiosity- why doesn't the patient simply have the other eye corrected to a level closer to the corrected eye???
Pete
Hi Jeff,
I'm a patient advocate and will always bow to experience. If you've slabbed a sv with success, that's great! Maybe I'm missing something. What happens when the person looks away from the oc above the slab-off? If the person holds his head straight and looks up, wouldn't he see double?
This is an extreme case! I'm with you Pete, why not just get the poor guy back in the normal zone. Acts of God are one thing, but a person can get too creative with monovision!
CJ,
You try to play the highest % of use theory, you just look straight ahead and converge more then looking upwards, BUT there are a few tricks that can be done to help there as well. :-)
Pete,
The main problem here with this RX is the powers and the way we can "manipulate" the images. JRS was right on with those numbers (%)I kinda rounded off the plus side and didn't throw the cylinder when I did my calculation so I was off slightly (give me a break :)) ..BUT the problem is with the powers involved using base curves won't get us close enough to a comfort zone for the person wearing the lens.
Even if I bumped it upto a 4 or even a 6 BC in that high minus I still only gained around -1% and You can , by dropping the plus side only gain a .25% or so in the OD (not worth doing)
So you have to solve this problem another way.. like I said the best option for this paticular RX is slabbing :)
You might also try using some vetical imbalance and cutting the degree of slab to help with the cosmetics and to help with the problem CJ mentioned "what about when they look up?" ...all in all it's a pretty tough RX and the optician better know how to trial frame or work well with the refracting OD to get it correct...
Optically, cosmetically and acuity wise I still think in this case a slab is the answer. WITH the possibilty of inducing a little vertical imbalance to help with the distant field..
Sometimes we lab rats can do a few tricks but with this RX the problem's can't be solved as easily with our bag of usual tricks.
Jeff "Tricks are for Kids..oh and lab rats" Trail :)
Okay Jeff,
I must be a sadist (or is it a masochist? oh well), because I actually put a +4.50 lens on my OS and then corrected things with a pair of glasses with the demo in the OD and a -4.25 on the OS...
Yeah, I notice the vertical imbalance- but I also notice that it wouldn't take much effort to "get used" to looking through the center of the lens (I'm doing so as I type :) ). As we've both pointed out, the real problem is probably the difference in image size, anyway.
I just can't believe anyone would voluntarily put themselves into the refractive state this gent seems to find himself in. I'm a fan of slab myself, but at the very least I would recommend trying a press-on prism first to "test the waters" on this pair. I always test reaction to slab by using a press-on first.
Pete "you may be right, I may be crazy... but it might just be a lunatic you're lookin' for" Hanlin
Like I said, what the patient needs is a good well fitted well managed hard contact lens or two.
The imbalances and centers and images sizes and prism will all become a moot point.
Chip
Chip,
I could never figure out this line of reasoning, "just give'm contacts".. BUT what happens if the get an infection? Or, for whatever reason, can't wear the contacts?
Do they just go without corrective lens? To me I think, if anyone, ever needed a backup for emergency this guy is the prize candidate.
When I first met my wife she went along with your theory about "oh I just wear contacts" ..now the problem is she is -8'sh OU and had never ran into a problem before. Needless to say you would know what a person with that strong of an RX would have to deal with if they were uncorrected.. which did happen, at work she got into some fume stuff that contaminated her contacts and irritated the eyes.. So there she sat with no correction :) .. I had to take her a pair of glasses to get her home.
Needless to say she had to learn the lesson the hard way (she is the one that is HARD headed not me
Depending ONLY on something as invasive as contacts only is a mistake in my book.
Jeff "can't someone else take my side in this thread...I feel pretty left out on this side of the isle" Trail :)
I am enjoying the arguments.
This can really take off and never land.
But, since we are all rendering opinions, why not take into consideration the likelihood that the poor sucker is suppressing the eye with least magnification. And, further consider that the refractionist may be detached from reality or is a recent graduate or may be even an ophthalmologist or, God forbid, an OA, trying to achieve emmetropia in the face of eminent amblyopia caused by an anasomatropic condition, and that no matter what we do to give the patient clear, comfortable, single, simultaneous, binocular vision . . . it ain't gonna happen!
Never heard of putting a slab-off on SV lenses,grind or reverse; where would it placed ? Getting the BC as close as possible would allow the patient to adjust to power difference better. If he or she experienced double vision, I would think that the OD would be in a better position to prescribe the appropriate amount of prism needed.
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