Equithin/ Prism Thinning

**Mikol** · 07-03-2001, 08:37 PM

Need an answer from the Lab Rats...........

Have been noticing lately that I am getting different amounts of base down prism in my PALs . The Rx is not to different from last years yet the lenses and prism thinning are all over the board. We dispense Essilor, Sola and Zeiss..What gives? Is my lab sloppy or are the manufacturers giving different data per their own lens design? I am now telling the lab how much prism to induce .........is this now the norm? your input will be deeply appreciated.............

Thanks in advance

Mikol

**Jerry Thornhill** · 07-04-2001, 10:29 AM

Mikol,
In the days when we were doing it together we had a set amount of prism to grind per add power and we didn't really consider anything else.

Things have now changed and computer programs and frame tracing have made things a little different today.

The amount of prism ground depends on the distance correction, the add power and the height of the fit. Because of this two Rx's for the same person in different frames may have different amounts of prism ground. The higher the fit, more prism is required to balance the thickness. The lower the fit, less prism is required to balance the thickness.

Jerry

**Mikol** · 07-04-2001, 02:39 PM

Jerry,

That does not really help. I understand what you are saying and thank you for your post but let give you an example ....

Rx
OD +.50 Sph
OS +1.25 Sph

Add +2.00
lens type: Panamics

I get lenses for this guy with 2 diopters base down and he is not happy with the lenses. I check what he is wearing and he has the same Rx in Zeiss Gradals but with .5 diopters base down. His Sunglasses are KBCO polarized progressives with .75 Base down.

All three frames are about 46mm with a B of 35.........So the frames are very close in size and I think it is just weird. If the frames were really different in size I could rationalize the differences.

Thanks for your help

Mikol

**JRS** · 07-04-2001, 04:00 PM

The original calc for prism thinning was 66% of the add power. So in your example; between 1.25 and 1.50 BD on a 2.00 add. Jerry Thornhill is correct in mentioning that the 'rules' can be more complex - although the ability to use that is lab computer system based.

I would guess that the other 2 pair do not have prism thinning or that lab used a fixed value/percent < 66%. The small amount in the Gradal pair is probably built into the design, and the KBco sounds like a lesser use of the old rule.

The one thing you did not mention, was did you order these (Panamic) uncut, and did all 3 come from the same lab?

If uncut, the lab may have used a B > 35mm (without a trace) and if the software is calc'ing to equalize the vertical, you could end up with over-compensation - or it could be the perfect equal thickness. Hard to tell w/o the lenses in front of me.

**John R** · 07-04-2001, 04:00 PM

We work on 2/3 rd of the add power as the amount of thinning prism. So a 3.00 add is 2.00 dioptre of prism. If you are getting diffrent amounts of prism then i would ask the lab making the lenses why?. I have not heard of diffrent types needing more or less thinning prism.

**Jeff Trail** · 07-04-2001, 06:02 PM

Mikol

With that example you gave, the one with the .5^ down prism thinning WAS used the one with the 2^ it was not...

By the way that was checking on the 180 right? and not from the fitting cross.

Really there is no "rule" as much as fepends on the ;ab and the experience of the lab guys running the lens. I know a ton of other lab owners and talking to them it depends on the "where and how" they usually do things, some of them only do it if it is requested on the order form.. while others do it depending on the lab guys running the job. I know I seen some others in this thread and the lens companies used to hand out all the tech sheets and it usually recomended prism as 2/3 of the add power.. in a lot of cases with the smaller frame sizes we use now this ended up being a little to much..I use two rings...a 1.00^ for adds above the 2.25 and a .75^ for below ..it makes for more uniformed jobs and far less confusion if say 7 months down the road one of my accounts needs only one eye remade, and it's uncut, they do not have to try to figure out where it is falling or send me the pair to "match" OC's since I use that formula for all my PAL's. Now if we ever get back to the days of 60's and large sizes coming back in style we'll have to readjust that prism but till then my system works well...

One other thing to mention is you have to consider if the design is sem-aspherical or aspherical, in an aspherical design you do NOT use prism thinning.

I would say from what you are talking about it comes down to SLOPPY lab work, no more no less ... and I can bash wholesale labs all I want since I own one!! :)

Jeff "grind em if ya gott'em" Trail

**Darryl Meister** · 07-05-2001, 10:24 PM

Hi Mikol,

Many of these responses have made several important points in regard to prism-thinning. Just to summarize the main points...

Originally, many manufacturers made prism-thinning tables for labs to use, which were generally based upon 0.6 to 2/3 the add power. These tables were designed to approximately balance the top and bottom thickness differential for a typical uncut lens blank -- particularly for plus-powered lenses.

However, this was only a simple and convenient approximation for labs, and did not take into consideration all of the factors involved that ultimately affect lens blank thickness. If the laboratory is using newer, more sophisticated software, the program is probably configured to consider several factors to balance the thickness differential exactly: PAL design, add power, distance Rx, fitting height, PD, frame size, etcetera.

The exact method employed by software calcs will generally produce optimum results. The overall thickness will be minimized for every power and frame size -- not just uncut plus lenses. Even minus lenses may be improved with some prism-thinning, depending upon the fitting height, frame size, etcetera.

Studies have shown that wearers can generally tolerate up to 2 or 3 prism diopters of prism-thinning. However, it is important that you match prism-thinning when replacing only one lens of a pair. There are some other visual effects, related to reflections and such, but they are generally insignificant.

Best regards,
Darryl

PS
Even if all other parameters are equal, the progressive lens design may still affect the prism-thinning amount computed. The sag differences across the vertical meridian of various lens designs will differ -- particularly between a spherical design (i.e., hard distance zone) and a highly aspheric design.

**Mikol** · 07-09-2001, 09:41 PM

Thank all of you who reponded....I now have a better grasp of what I was unsure of......

Thanks again

Mikol:cheers:

**Corey Nicholls** · 07-10-2001, 01:48 AM

Finally, some people who know what they are talking about!!!

This is the first time in all my fifteen years in the industry that someone can have a conversation about the matching of prism thinning for a single lens replacement, without the lost looks of "What the hell are you talking about?".

The amount of people here, even the most qualified who have no idea about prism matching is terrifying! This even goes for some lens reps who are supposed to be qualified "Lab Rats".

**John R** · 07-10-2001, 02:34 PM

Yep Corry you are so right.
It's very important that if ordering a half pair of varifocals that you order with the correct amount of prism, better that you specifey the amount rather than rely on the lab to guess it right, as has been pointed out here, not everyone works to the same system anymore. Save's a hell of a lot of trouble in the end.