the problem w/edging a lens w/12.5 prism OU...

**eyecarepro** · 12-02-2001, 01:59 PM

Okay, I need to vent here: I went into work this morning, ON MY DAY OFF to see how my boss was going to solve the problem with edging these lenses with 12.5 diopters of prism IN EACH EYE. (NOT 6.25 in each, a total of 25!) I may sound a little naive, but I have never seen such a monstrosity of a lens as this one with THIS amount of prism. I DO understand why this poor lady has all this prism, but that's beside the point.
I had tried to edge it myself, last night, but the [Santanelli] machine I was working on wasn't having it. My edger parts terminology is poor, so I'll have to describe it: the two pieces that come together to clamp the lens tight as it's being cut and beveled- they did'nt close all the way so that it held the lens flush. The edge was SO thick from all the prism that it closed, but had a big gap between the clamps and the lens. I don't know how much sense this is making but I'll continue. My boss, seeing this, came up with this brilliant idea, or so I thought at the time. He stacked about a dozen leap pads on the concave side of the lens where the gap was and re-chucked the lens, suprisingly the clamps came together flush around the lens, so it had seemed the problem was solved. We did this for both lenses and they cut out nicely, as dreadful as the finished product looked. The bevel was a little crooked, but it still fit the frame okay. Obviously there was going to be NO temple closure with the amount of prism induced edge thickness, so we decided to roll down some the remaining edge just enough to get SOME closure. Before we did this, boss man checked the PD and bifocal segments to make sure everything was cut according to specified. The PD was 10 mm narrower than specified! At the time I couldn't figure out was on earth could have caused such a blunder. Of course his answer was that I didn't enter something in the edger settings right, the PD (near for bifocal), or put it on the correct setting for bifocal. Knowing this job was as crucial (and expensive) as it was, I checked and double checked everything before hitting the start button for the edger to go on ahead and edge the lenses. Everything was set up PERFECT.
After much consideration, later on today, I came to the conclusion that because the edger is NOT set up to cut lenses of this magnitude, putting all those leap pads on the backside of the lens like we did threw off the machine's ability to edge the lenses to the PD we desired. Even though it seemed like a good idea at the time, it actually tilted the lens off the correct angle in which the machine was set up to cut lenses at default.
I know this is a VERY detailed and lengthy story IF it even makes any sense. However, I really need to reach some personal resolution as to WHO'S blunder it really was. If there IS anyone who can make sense of this, I'm WIDE open for any and all feedback. I'm very frustrated over this whole mess. It will be very time consuming to remedy it.

**Rich R** · 12-02-2001, 05:14 PM

Hello eyecarepro, I think I understand some of the problem and something that would throw the pd off would be during the finish layout blocking, when you have a lens with that much angle, put that leap block on there and then measure from back side, I have had this problem when doing bifs, with much vertical prism and the seg heights are a couple mm differencd, one too hi, one too low, now I measure and reblock with some compensation, solves my problem, may help...
Rich R

**Mike Fretto** · 12-02-2001, 05:14 PM

I'm just thinking out loud but if the block wasnt placed 5mm per eye narrower than it was supposed to be at layout its hard to believe the edger would move the lenses in 5mm because of prism.

**chip anderson** · 12-02-2001, 05:35 PM

Know a pediatric opthalmologist who is good a muscle surgery?

Chip

**LENNY** · 12-02-2001, 08:22 PM

I think it came from blocking!
Do you use a blocker?
If so the chuck probably landed in the vrong place because of the angle of the lens on the table.
Are the prisms vertical?

**shanbaum** · 12-03-2001, 05:51 AM

Every finish blocking system available, except for two, has a problem we call "through-power distortion". This arises from the designs of the blockers, which in every case (again, except for two), incorporate the optics of the lens being blocked into the alignment optics - so that the power (especially prism power) of the lens causes the alignment target to deviate from its actual position. In short, the operator looks at the alignment target through the lens.

The two blockers, the designs of which avoid this error, are the Weco CAD 2000 style (I believe they have some other designations these days) and the Gerber Coburn Step Two.

Unless you're using one of these two blockers, I'd guess that's the source of your PD error.

If you are using one of them... never mind.

**Jeff Trail** · 12-03-2001, 12:33 PM

The key to this is that "angles" is how we create prism. The second factor is the power of the lens. Remembering that the angle of the lens also moves the optical center as well by moving the angle of the lens (tada, remember ray tracing?)
The second factor to that problem is the way the power of the rx will effect the way it interacts with power/prism ratio. Going back to basic optics, you have a minus rx and you want prism out than you decenter in to move the oc out. Reverse that with plus power.
You "****" a lens in a frame by that "crooked" bevel and you are moving the oc.
When you lay out the lens in the lensometer you need to allow for that bevel placement, the lensometer is taking it as if you tracked the lens the way it is sitting on the lensometer. If you do NOT do it than your end result will not be correct (the 5 mm difference OU)
The other thing you really should do is always crib a high prism job before checking and blocking and you will get a more precise layout.
Last but not least when you start tinkering with leap pads and angles you should place the leap pads so it does NOT change the angle of the lens through the process. The trick is to "fold" the leap pads and use it to layer at the same angle NOT the whole leap pad flat against another. This little trick works great for high adds and Lenticular's as well.
By using half pads you are working the lens like a wedge, and since all the surfacing was done taking that the front surface was parallel to the block that to reproduce that amount of ground prism than the front surface of the blocked lens in the edger should ALSO be parallel to the chuck.
Does it work? You bet, I do tons of high prism jobs and I have no problem at all and if I want to track the bevel other than around the face of the lens I compensate while surfacing. The ole' do it wrong to get it to come out right in the end. :-) Just have to remember that the power of the RX will tell you on how to compensate.
Stacking a bunch of leap pads on top of each other just does not work because as you found out by changing the angle of the lens from generating to the angle it beveled at you neutralized or added prism (depending on the power and angle) If you "stack pads you need to do it in a way that you gain in thickness (for chucking purposes) but still keeping the correct angle. Use that folding of leap pads and staggering the stack and it will work ust fine.
I have Briot 6000 and do it all the time and It works for me.

Jeff "funny how it all comes back to the basics" Trail

**Clive Noble** · 12-03-2001, 02:52 PM

I'm glad I read this thread tonight, 'cos tomorrow morning I'm making a pair of Franklin Bifos, (yet another pair) with 10 dioptres prism in each eye for distance.

I'll try Jeff's folding leap pad trick, sounds good.

**eyecarepro** · 12-04-2001, 11:51 AM

Actually, we had faxed in the order to DBL and had THEM do all the hard work. Boss man did the ole "5 in, 5 below" routine, and I blocked it up later for edging the way I would have any other lens, seeing as the prescription was already ground in- was not under the impression I was needing to block it up for the edger any special way than the usual. I had no worries on the surfacing end. As usual, DBL did a phenomenal job on this lens. My only concern was to block it up, and edge it down. I had not anticipated anything out of the ordinary occurring. I have not been at this QUITE as long as some of the rest of you all. I'm still honing my surfacing terminology. Any time we have a job of this magnitude we will usually send out to have it surfaced and we will cut additional costs by edging it in house. The job was obviously no good so we sent the whole kit and caboodle (frame AND order sheet) to the lab to have them surface AND edge it to eliminate any more problems like these. Incidentally, I feel pretty foolish about this whole thing. But it HAS been a learning experience. Unfortunately, I'm still a little puzzled over the whole thing and why it turned out the way it did. All your thoughts and ideas on this were appreciated. I was only able to follow some of what was suggested, however, what with my limited experience with more detailed jobs like this that require more attention. Hopefully I will gain more experience and knowledge as more of these kinds of jobs come out of the woodwork.

**Jeff Trail** · 12-04-2001, 01:22 PM

eyecarepro,

One thing that you might find pretty helpful is to take a few classes via the web. One of the regular posters on this site is an instructor with the school (Laurie) A couple of classes you might really find interesting is Geometric optics.. goes into ray tracing, and a ton of the lens maker formula's and prism and slabs and on and on.
They also offer lens theory classes, I think it's four semesters. You get most of the info on the WebCT site and via tapes and handouts.. plus a ton of tests :-)
If you are not going for the degree but just a few of the classes to bump your knowledge in lens theory than they would probably let you get by with phone registration and not have to go to the college.
You will learn a ton of the formula's and basic formulas in optics. How the lens reacts to images etc. etc....
If you have any other questions or need the formula's let us know, one thing about this site it's full of optical fanatics.
Once you grasp how angles and movement neutralizes prism I think it will all click in.
The basic formula's that were in play here were Prentices rule, with or against motion (depending on the RX) ray diviation from angle (that bevel moving around)..plus that pancake stack of leap pads :-)
Once you get down how angle of lens surface to frame moves the OC than you'll get the hang of it..I think your only mistake was you looked at the chuck as the problem and kinda lost sight of the lens angle.

One thing was that you asked, that's a good step in the right direction. At times I know we tend to go off on tengents here and since we all are at different levels of knowledge we tend to forget to fill in the blanks at times.

Look up Laurie profile and drop her a E-M if you want the info on the school, it's well worth the investment in my opinion. BTW I think there are a few Lauies on here so it's the Laurie that is in Tampa :-)

Keep hacking away...

Jeff "we all started somewhere" Trail