Equal base up prism in PAL Dist OCs - Okay?

**William Walker** · 11-17-2006, 01:53 AM

I received a job today from the lab. Easy Rx - spherical, OD -6.25, OS -6.50; 2.50 add.

In each eye, when I am looking through the distance OC, the target comes in clearly at 5 D Base Up through the reticle. I examine the other lens, same thing. There's no vertical imbalance to speak of, so I can't reject it for that, but if there's 5 D BU, the patient is going to want to look through a lower portion of the lens for the distance Rx to be coming in through the OC, which won't work, because that's where the progressive starts! Is there any kind of ANSI tolerance to quote for this?

Thanks!

**Leo Hadley Jr** · 11-17-2006, 07:18 AM

Hi WW,
When looking for prism in a progressive you must look at the "prp" which is usually a small dot right under the cross. If you look there it is probably OK.
The prism is a result of prism thinning.
If you want to read the lens on target through the distance, those lenses would be thick as a brick.

**William Walker** · 11-17-2006, 10:59 AM

Thanks for the reply. I did look at the PRP to double check for VI, and there was none, so I know that that specific part is in check. I just wonder, for someone with over 6 D of minus power, what will they feel as they look out of the lens with 5 D BU in each eye?

**Darryl Meister** · 11-17-2006, 12:39 PM

Where did you measure the lens for the distance prescription? If you verified the power in the distance checking circle, which is where the Rx is normally verified, keep in mind that this is generally centered about 8 mm above the prism reference point (located at the center of the lens blank). This isn't necessarily where the patient will use his/her distance vision, but is instead a convenient point to neutralize the prescription without the effects of unwanted power from the progressive lens surface.

In the absence of prescribed prism or prism-thinning, the optical center will usually be at (or near) the prism reference point. This means that, in the distance checking circle roughly 8 mm away, the prism induced for a -6.50 D lens is 0.8 * 6.50 = 5.2 PD.

Unless you're seeing a significant amount of prism at the prism reference points, I wouldn't worry about it. Otherwise, this situation is no different from the prism induced in single vision and flat-top bifocals.

**EyeFitWell** · 11-29-2006, 10:29 AM

BU OU is cancelling prism...right? Isn't that basically the same as everyone walking around wearing 5D BD OU?

**Darryl Meister** · 11-29-2006, 12:24 PM

Yep.

**EyeFitWell** · 11-29-2006, 12:29 PM

:bbg: Guess that tuition is starting to pay off!:bbg:

**Darryl Meister** · 11-29-2006, 12:42 PM

Education usually does. ;)

**TPL** · 12-06-2006, 10:20 PM

Just for the record prism thinning {equathin} is used in plus prescriptions only. the theory behind doing it was by grinding bu prism of equal amounts{1.5} in each eye would reduce the thickness in the lenses and being that the direction was the same in affect it had no affect on the eye.

**Barry Santini** · 12-21-2006, 07:45 AM

Originally Posted by TPL

Just for the record prism thinning {equathin} is used in plus prescriptions only. the theory behind doing it was by grinding bu prism of equal amounts{1.5} in each eye would reduce the thickness in the lenses and being that the direction was the same in affect it had no affect on the eye.

Its a shame that execs never took this approach in moderate and stronger plus powers. When I add "prism-thinning" to an exec order, the lab charges me, and no lab manger I've ever spoken to understands the idea.

I think most Lab people are under trained, especially with today's computer-based processing found in the lab.

Also..prism is prism...and prismatic *imbalance* is somethin' else....

My two cents

Barry Santini, ABOM

RT · 12-21-2006, 01:46 PM

Guess you haven't spoken to the right lab managers, Barry. We routinely prism thin execs for many accounts, at no extra charge.

**EyeFitWell** · 12-21-2006, 05:23 PM

Originally Posted by TPL

Just for the record prism thinning {equathin} is used in plus prescriptions only. the theory behind doing it was by grinding bu prism of equal amounts{1.5} in each eye would reduce the thickness in the lenses and being that the direction was the same in affect it had no affect on the eye.

I'm confused, now. I thought that prism thinning was also done on a minus lens, only with BD prism. Can anyone confirm?

**OPTIDONN** · 12-21-2006, 05:26 PM

Originally Posted by EyeFitWell

I'm confused, now. I thought that prism thinning was also done on a minus lens, only with BD prism. Can anyone confirm?

I have never seen it but I have heard some opticians say that they do it. I can't imagine it making too much of a difference.

**Darryl Meister** · 12-21-2006, 05:26 PM

It depends on the lab software, and how it has been configured. It is possible to prism-thin either plus or minus lenses though.

**akeroptical** · 12-27-2006, 08:16 PM

My understanding is that prism thinning is used to balance the added thickness that is created in the bottom of prog. lenses from the ADD power.

I could see this being done on plus and minus Rxs but with a more obvious benefit to higher plus Rxs in high add powers.

Am I thinking correct?

**Darryl Meister** · 12-27-2006, 08:27 PM

Everything you ever wanted to know about prism-thinning, and a lot you probably didn't: Prism-Thinning Article.

**TPL** · 12-27-2006, 09:27 PM

I recently read your article and thought it should be must reading for any individual in the opticle field. The confusion and misconceptions on this topic are common amongst the opticle professionals.

I enjoyed the article and are recomending it to many people!

Thanks for the time and effort put into it!

TPL