# Optical Forums > Ophthalmic Optics >  Bi-Concave

## Mike Fretto

We have an order for -19.00 for a two year old child:( . I would like to attempt grinding a biconcave in an attempt to 1)make it look a little better and 2) just plain make it easier, weve had problems in the past with higher powers flying off the finer even after adjusting the stroke down. My question is this, how steep can you go when grinding the front surface? I'm not finding a semi-finished bi-concave poly sv blank to use so I'm forced to try and make one.

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## Joann Raytar

Mike,

I think some equipment companies used to sell special lap table adaptors for high minus fining.  I am not sure if they still do though.

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## Jeff Trail

>>1)make it look a little better and 2) just plain make it easier, weve had problems in the past with higher powers flying off the finer even after adjusting the stroke down. My question is this, how steep can you go when grinding the front surface?<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<


   Mike,

     A lot will depend on the type of blocking system you are using as well as the blocks. I found that the one step wax really has a hard time working with this type of grinding the wax is just not tough enough and the blocks pop off.
     I usually use the alloy and use a small disposable coburn block.  I have not had any trouble going as high as a -6.00 on the front and having it come out correctly and no run problems. 





>>>>> I'm not finding a semi-finished bi-concave poly sv blank to use so I'm forced to try and make one.<<<<<<<<<<<

    I found this part a little confusing? Are you saying you are looking for a "molded" lens that is for biconcave? I hardly ever use those I just take a regular stock blank flip it around and run it backwards.. depending on the lens maker you can get a wide variety of "curves" to start from. Oh BTW if you do it this way better make SURE you coat that inside surface, that will become the "front" surface when you flip it around. most are coated but not ALL.

   Another option to consider is this would be a very GOOD case to use tri-vex (Trilogy,Pheonix).. less hassle than poly and not as easy to scratch ;) One other thing I found that worked well is to find the edges of the "block" where it will mount and I take leap pads and place on the lens at even points that fall half in and half out of the block..this is nice to ensure the lens block stays clear of the lens surface. I put the leaps on and than place the tape over it..seems like and odd ball way of doing it but it sure helps... one other thing I like to do is make sure I remove as much excess blank as possible from the edges before even putting it onto the cylinder machine and that helps remove some of that "rocking" you get. Last but not least (may be the "craziest") is when I was grinding one that had a -12 on the front I actually strapped the lens to the pin holder!! I left the blank width bigger and took a dremal and at an angle popped holes through the lens (top and bottom) on both sides..placed the lens on the lap..ran the wire up over the axis pins and through the opposite holes and tied it off tight as possible... pain in the butt but after having the pins pop out of the block about a dozen times and fighting with it, it worked..a little time consuming but sure beat eating lens after lens.

     Think about that tri-vex solution.. the benefits of poly but less of a hassle to produce, specially in a specialty job such as this.

Jeff "love the tough stuff" Trail

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## Jerry Thornhill

Mike,
I have done fronts up to a -12.00 on the cylinders with no problems. It may depend on the cylinder machine. I am using the old standby 5056 Coburns. I adjust the stroke on top and also on the bottom. I try to get the smallest amoubt of travel that I can.
I have done backside to a -18.50 with no problems.

I agree with Jeff about cribbing the lens as small as possible. Material is your choice if you normally process Poly with no problems this should work out just fine.

Another solution except that it is for a chuld is that Optima has a 
-6.00 Base in 1.66.

Good Luck,

Jerry

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## RT

The act of grinding a minus curve on the front of a lens is no different than grinding a minus curve on the back of a lens.  As you've noted, grinding real high back curves has its own set of problems, so using minus fronts may help you solve that.  The problem comes from being able to block the lens that now has a minus front, since most blockers pretty much assume that the front curve of a lens is plano to plus.

The first problem will be that the more minus the curve on the side of the lens that contacts the chill ring, the greater the chance that the lens will not sit correctly, causing you to spew alloy or wax.  You'll also cause a thickness error, since the lens is now contacting the outside of the chill ring, and not the inside (effectively changing the ring diameter).  Be sure to allow some extra thickness.  I believe the theoretical limit for blocking minus front curves on Step One is between -4 and -6 diopters--above that, and the lens won't sit on the chill ring right.

The second problem is filling the larger gap between block and lens correctly.  That's not too much trouble with an alloy blocker, although the sheer volume of alloy required suggests a lot of heat to dissipate, which could cause problems with poly.  With the Step One, be sure to use the "Minus front" block, which is designed to reduce the volume of wax used.  If you trap an air bubble, you'll probably have difficulty getting a good lens due to aberrations.  We use Step One for all our non-glass work, and have no difficulty with deblocking on extreme jobs.  However, variations in machinery used can have a huge effect on the amount of "beating" the lens and block take as you get into extreme curves.

Cribbing, as Jeff and Jerry point out, is always a good idea.  Among other things, it makes it much easier to deliver polish to the center of the lens during polishing.

Jeff's solution of just turning a standard blank around is probably, however, the best.  Among other things, its doubtful you're charging enough for the job to warrant surfacing it twice.  Jeff's Trilogy/Phoenix suggestion is also good, because of some of the superior properties of that material.  However, with lower index of refraction than poly, using Trilogy/Phoenix means your backside curves will have to be slightly steeper, which could put you back outside your "comfort zone".  We did a +13 with -14 cyl out of Phoenix not too long ago, and found it to be easier than poly to process, and the patient found the optics to be much better.

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## John R

> _Originally posted by Mike Fretto_ 
> *We have an order for -19.00 for a two year old child, weve had problems in the past with higher powers flying off the finer even after adjusting the stroke down.*


I take it you mean a -19.00 sphere with no cyl
Do you not have any old Sphere machines ? or know anybody with access to one. Surely there is somebody state side that still has one.. This would be very easy on one of these.

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## Mike Fretto

I will attempt to answer the questions asked then give an update.

1st) Jeff  I found this part a little confusing? Are you saying you are looking for a "molded" lens that is for biconcave? 

 Yes I was hoping for a semifin blank to start with and it had to be poly. When I couldnt find a  minus base curve to order I decided to make it, and yes we did backside coat the front before reblocking and grinding the back. 


We use the one step blocking system by Gerber/Coburn and found the minus base blocks RT mentioned. There was no problem processing these lenses today, thickness came out fine and the power was well within tolerance. I have never liked using the foam tools but for that high a power we had no choice. We ground a front curve of -3.00 probably could have went higher but I wasnt sure if there negative consequences to going higher either cosmetic or optical. Using Daryl's Optics lite program we determined that by grinding a minus base of -3.00 we only actually dereased edge thickness .9mm but reduced the inside curve 3.50 diopters which helped in keeping them on the finer. Anyway thanks for the input the original question was how steep can you go and I think you guys answered that thanks again.

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## Jeff Trail

Mike,

  Glad it all worked out :-) ... and worrying about the amount of negative base curve on the optics (as well as thickness) does come into play, but not as much at the curves you were dealing in.. the main problem is that a high myops is used to that "fish bowl" and if you neutralize it to much with off setting curves they tend to complain about tunnel vision.. I always try to get the BCA before tinkering to much with the optics by playing tricks with the curves.
   I found most people can tolerate up to the -4 to -6 base range without to much of a problem since they are used to that over warping of images... one other thing we try to tinker with is the bevel placement and the vertex power. Sometimes that can get tricky with the really high myopic RX (say the -20's and so). Sometimes if you get real technical and have a good surface program or a distometer you can make some adjustments in power by manipulating the vertex and even get a thinner lens :-)

Jeff "love these jobs better than cranking out .50 spheres" Trail

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## Joann Raytar

At an Rx of this power, would you folks recommend a bi-concave or a miodisk and why?  In similar cases with well fit frames I have also chosen bi-concaves but I had a customer a long time ago who was amazed at how much more clearer he saw through a miodisk versus his old Rx.

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## Jeff Trail

Jo,

   Myodisc and aphakics (aspheric or lenticular) do have a place ..BUT you lose full field at a greater degree.. mostly I try to take into account a few things, BCA and the age of the person.. say I have an elderly person who has needs that +14 with a 5 add than they can get by with an aphakic most of the time..but you have an active 35 year old they want as much visual field as possible.. in this case a two year old who is still forming optical than I would have really pushed the full field of going with a biconcave than narrow that field by using a myodisc. 
   I think it mostly comes down to using a little common sense and putting your optical knowledge to work when deciding on the correct design to use. Technically a number of designs would HAVE worked but I think Mike's choice was probably the best in this situation. Especially if this two year old is going through some visual exercises and working with a behavioral optometrist.


Jeff "myodisc make great paperweights" Trail

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## JRS

Given that theRx in question is for a 2-year old. And probably wearing about a 34 to 38mm (A) frame, I would have thought a myodisc would have been quite adequate. The field of view is usually around 38mm on most. Even with reasonable decentration, I find it difficult to believe the kid would have had any problems with field-of-view.

Just have to grind 1 side to complete the job.

Why did you discount myodisc on this Jeff? Just curious.

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## Darryl Meister

> _Originally posted by Jo_ 
> *At an Rx of this power, would you folks recommend a bi-concave or a miodisk and why?*


Hi Jo,

I'll preface this response by stating that high-minus lenses with plano (or near-plano) base curves perform better optically than lenses with concave base curves. So a bi-concave lens design will not provide a particularly wide field of view, and will also have greater minification.

I think that we should distinguish between a true myodisc versus a minus-lenticular design. Historically, myodiscs had relatively flat (plano) curves ground on the front, with perhaps some cylinder power. A plano carrier curve was ground on the back. Minus-lenticular designs, on the other hand, use a significantly concave front curve and a convex carrier curve.

If you are using a prefabricated Younger Blended Myodisc, it actually uses something more similar to a minus-lenticular lens design (with the edge of the bowl blended). Consequently, the Younger product also has a concave curve on the front. So, there is not much of an optical difference between the Younger Blended Myodisc and a typical bi-concave design.

However, there are some obvious physical differences. The Younger design will be thinner and lighter in weight, but will have reduced field of view because of the smaller aperture of the lens. If the Rx range allows for it, you might also consider a 1.66 high-index lens material (preferably with a backside aspheric/atoric design). This will get you a full-field design with less thickness and better optics than many bi-concave lenses. Of course, if children are involved you may opt for poly.

Best regards,
Darryl

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## Jeff Trail

JRS,

    A couple of reasons, one I like to think a little more on the safety side..I would have placed this RX in Tri-vex myself and not the poly..
    Second I have a cousin who has very high myopia, she was three when we made the last change (-15's touch of cylinder and small amount of prism) and I tried doing it a number of ways while working with a behavioral optometrist who is a friend of mine and while playing with puzzles and such as well as shape programs we found she had a far better vision in the biconcave combination. The myodisc was second and a regular set last... she also has a slight degree of amblyopia that needed corrected.
    with the myodisc she was moving her head far more or missing pieces when working the puzzles.. we also ran into some problems in depth of field in the myodisc. That one I could not figure out but she was having a slight problem. 
    I work with a number of low vision specialist as well as the council for the blind and in adolescents we found the myodisc just was not as effective.. of course "optics" is a changing thing and there were some cases that just the opposite worked but probably 4 to 1 in the single digits (1 or so to 10 year olds) we found that 4 to 1 ratio.
  Now in the late teens to early 20's it was almost just the opposite..probably about an even split, but we did find that they felt more comfortable in the biconcave slightly higher numbers than the myodisc. A lot depended on the best corrected acuity and power.
   I'm sure others probably found the answers some other way but all my tinkering this was about the best ways we came up with. 
   One of the odder things I found was at times we found that some people even liked the high curve (comfort wise) I lean towards the way the cornea shaped and following along the optical axis may have something to do with it but not real sure, that is the nice thing about dealing with kids (the real young ones) they have not developed a sense of "vainness" and acuity is far more important than cosmetics as I have run into in the puberty crowd :-)
   Well their it is.. I'm sure others have reasons that go against my reasons but from my experience is what I was going by....
   If anyone else wants to jump in feel free, I'm always open to suggestions and new tricks specially when dealing in low vision and children.

Jeff "always willing to learn a new trick" Trail

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## Laurie

HOW IS BEAUTIFUL BABY LAUREN!!!!!

(almost sounds like Laurie !)

: )

How many days is she now?

Laurie "I absolutely adore babies" Pierce

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## Jeff Trail

Laurie,

    The "lab rat" is doing GREAT :-) ..she is eating 4 oz. at a whack now (she wouldn't breast feed) .. I think we got lucky here she is only two weeks old and she is going to sleep at 11:30 and does not wake up till, usually 7:30 or 8 am. !! ... she has put on 8 oz. already  and we have went through about 5 rolls of film. Last night was her first official bath.. going through a lot of "first" here and me and Susi are loving every minute of it. 
    I have gotten a little behind in the WebCT stuff as well as work but it's just to easy to want to play with the new "toy" than do homework :-) .. Tell Bill I'm trying and I WILL GET to that latest exam in contact lens ... Thanks for asking and we are all doing well 


   OH and JRS and others.. I forgot to mention that kids are my "weak" point and when I do all that low vision stuff I lose my rearend from a "business" point of view.. I try every combination and just charge for one pair of lens :-)

   One other thing we have tinkered with is using PAL's and bi-focals in developing myops anyone else trying that stuff or tinkering with it? I guess having a lab it does not cost as much to tinker and the OD's I work with kinda help ease the costs as well and I am lucky enough to have a few low vision OD's and behavioral OD's that are friends who also have that "weak" point for kids and we try to help out the kids and not really looking for any profit. Nice to see not all people are looking to make a buck and are interested in helping out. :-)

   One other thing, I know I go around and around with some of my MD's about prism and dealing with RP and AMD and I found by trial framing that once you reach a point in redirecting images with prism that all you get is more "light" but do nothing for acuity...passing along an RX with 30^ of prism is not doing any better than using 10^ or so on the acuity chart. I even spent time sitting and working with them and showed them and they always tell me "well the text books say this"..I even yoke prism with people with AMD and had some really good results..

  Oh well, guess I just have love to tinker with low vision. Maybe I'm so "text" dumb I don't know things "shouldn't" work but they do..:-) If any body has any experiences in PAL's and bi-focals working with kids I would loke to hear how it turns out and the case history.. and dealing with AMD or RP if they tryed yoking what they got as results.. or thoughts on it...

Jeff "the rule I go by, no matter how crazy it seems if it works it's right" Trail

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## Rod Hendricks

When i read this thread, it reminded me of one of my favorite optical stories. A man, age 40, came in, the doctor examined him, wrote the RX and then he landed in my lap.

The RX was something like OD: -5.00-11.00 x 90, OS: -6.50-12.75 x 85 (numbers aren't right, but they're close enough). We went through the frame selection process. I noted that he was wearing bi-concave lenses, clocked the lens and ordered the glasses.

They came back from the lab, we called him, he came in and indicated the vision was good with the new ones but said there sure was a lot more change than he was expecting. I rechecked them, RX was correct, and advised that it was probably just that "getting used to the new RX" routine that happens often.

He left, was back in a couple days. I rechecked them, the doc looked at them, rechecked the guy, same RX. He had his old glasses with him (he was wearing them when he came in, instead of the new ones). Bottom line is I rechecked the base curve on the front and noticed there was some cylinder this time. Having cut my teeth in a plastic lens surface lab, doing practically nothing but aspheric lenticulars and myodiscs, I never dreamed of checking more than one axis for base curve the first time.

The lab that had made his existing glasses had put 5.00 diopters of cylinder on the front side, the rest on the back side. I explained to the guy that while optically, that would work, the problem he was going to have was right around the corner when he needed bifocals (remember, the guy was 40). A bicylindrical lens couldn't be fabricated with a bifocal segment in it. I explained to him that it wasn't a matter of whether he was going to have to make the change, but when. He decided to have us remake them again, as a bicylindrical lens, against our advice. We remade them and he walked out smiling. I no longer work for that MD anymore, so I don't know what happened a few years later when the guy's "arms weren't long enough anymore".

Again, as an old lab guy, I understand why one would make the bicylindrical lens, it's just flat out easier to surface, cut tools, keep on axis, generate, whatever. But what a nightmare that created for the poor dispenser (me, in that particular instance) and for the patient himself. Sometimes "easier" isn't always "better" for the patient.

Rod

http://www.eyevillage.com/index.spml
If it's eye-related, you start here.

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## nixg

tell me which blank i have to use here 0 base or 12 base for making -1600 no.
in biconcave fashion






> JRS,
> 
> A couple of reasons, one I like to think a little more on the safety side..I would have placed this RX in Tri-vex myself and not the poly..
> Second I have a cousin who has very high myopia, she was three when we made the last change (-15's touch of cylinder and small amount of prism) and I tried doing it a number of ways while working with a behavioral optometrist who is a friend of mine and while playing with puzzles and such as well as shape programs we found she had a far better vision in the biconcave combination. The myodisc was second and a regular set last... she also has a slight degree of amblyopia that needed corrected.
> with the myodisc she was moving her head far more or missing pieces when working the puzzles.. we also ran into some problems in depth of field in the myodisc. That one I could not figure out but she was having a slight problem. 
> I work with a number of low vision specialist as well as the council for the blind and in adolescents we found the myodisc just was not as effective.. of course "optics" is a changing thing and there were some cases that just the opposite worked but probably 4 to 1 in the single digits (1 or so to 10 year olds) we found that 4 to 1 ratio.
> Now in the late teens to early 20's it was almost just the opposite..probably about an even split, but we did find that they felt more comfortable in the biconcave slightly higher numbers than the myodisc. A lot depended on the best corrected acuity and power.
> I'm sure others probably found the answers some other way but all my tinkering this was about the best ways we came up with. 
> One of the odder things I found was at times we found that some people even liked the high curve (comfort wise) I lean towards the way the cornea shaped and following along the optical axis may have something to do with it but not real sure, that is the nice thing about dealing with kids (the real young ones) they have not developed a sense of "vainness" and acuity is far more important than cosmetics as I have run into in the puberty crowd :-)
> ...

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## HarryChiling

It is the 0 base lens.  You are going to surface both sides of the lens.  I agree with an earlier post it is much more easier and convienient to surface a spherical front surface.

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## nixg

Suppose i have 0.50 base cr 39 blank with 13.00 mm edge thickness and CT 7 mm how to surface -15.00 power on that lens







> It is the 0 base lens. You are going to surface both sides of the lens. I agree with an earlier post it is much more easier and convienient to surface a spherical front surface.

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## shanbaum

> Suppose i have 0.50 base cr 39 blank with 13.00 mm edge thickness and CT 7 mm how to surface -15.00 power on that lens


Most roughly-flat CR-39 semi-finished blanks have roughly-flat back surfaces as well, so you should have around 18-22mm to work with. Poly blanks, I know, at least used to have steep back curves on the flatter semifinished blanks.

But if your flat blanks don't have sufficient thickness, use whatever will work. If a high diopter semifinished blank is all you have that has sufficient center thickness, have at it.

I've never tried it, but I don't see why you couldn't make a myodisc in this way by grinding a steep minus curve into the steep plus finished side of a blank (using that as the _back_ of the finished product), and grinding a flatter minus curve on the "back" of the blank, used as the front. 

You'd want to do the flatter curve first, unless there were lots of cylinder, which might make it difficult to block on that surface to do the other side. I'd certainly put the cylinder on the flatter surface if possible.

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## Darryl Meister

> You'd want to do the flatter curve first, unless there were lots of cylinder, which might make it difficult to block on that surface to do the other side.


Particularly since cylinder will distort the shape of the bowl into an oval with its major axis aligned with the cylinder axis.

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## William Brown

:idea: First take the rx go to the computer and figure out the best possible configuration for say -16.00 actually this lense could be done either way.A small minus front or flat-I like poly and make my front curve first and hard coat it then turn it over and put the rest of the minus on with a 1.5 center then put a myo or lent edge making the myo about 4mm under the vertical and hard coat that side. If you have a cylinder on the myo side put cylinder at oppisite axis- it will make the field even-ROUND this is my kind of rx work. don't like isoconic if I can stay away from it but it's in my bag of things to do.I'm lucky I have a line of sphere machines. Still use a 390 shuron and a 113 with the shims Plano-20.00- oldfield lap cutter still being used-calipers and a ruler is all I need. Can you tell I love my work.

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## William Brown

Want tough work advise you see Dr. Siwoff or Dr. padula they both have web. sites and are very precise specialist's in this field when talking optics you need to talk to dr. siwoff he has far more research on the mental and optical aspecks of this business. What he is working on will blow your optical mind. I may an apprentice but I know who to contact for everything in low vision.Myo-dics and such are minor to their rx's. Lab. rat

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## Thomas

In the lab I work at we do nothing but specialty work such as this. Fullfield myos, blended myos and bi-concave blended myos everyday. Not to mention all the saddlebacks and the very high prism (25 diopters is the highest I have seen so far) Lots of times we simply use the backside of the blank as the front. Works very well, saves time at the generator and cyl machines. We made a pr -28.00 and -27.50 blended bi-concave myos a week or so ago that was sent out the door the same day it was called in. 

It later came back with the frame for edging. :cheers:

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## William Brown

I'm really glad to see a fast return on special rxs-great job-I work alone and only part time on specials because I'm also the production manager-maintainence man-delivery car maintenence man-but I still like to dabble in it. I like the three step process of tools in the blended myo-disc-If I get overwhelmed with special work I'll think about your fast service. Bill

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## William Brown

To start with you make your own front curve even if its high index. You can hard coat that surface.Use plastic for the abby value and from the front curve say -2.00 or three or four to get the front curve to make a full field. I've done bi-concave for the rx part and a bi-convex for looks on rx's over -30.00.To me this is fun.I love a challenge and turn nothing down until I have exhausted all possibilities at my own expense with many blanks. And I love to talk optical problems.Call me at winchester Optical 607-73-44251 It's Bill Brown Special surfacing.

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