Thinning prism

**VVizard** · 05-18-2006, 08:47 AM

Hello, everybody

I would like to ask you what is the common practise in dispensing lenses (especially progressives) regarding thinning (cosmetic) prism. Do you have different policy for plus and minus lenses? If so, why?

Thanks.

**OPTIDONN** · 05-18-2006, 09:48 AM

Usually the lab calculates prism thinning before generating the lens. A good rule of thumb to determine the amount of prism thinning in .6 x the add power. This is only done on plus powered lenses. If you look in the file section of Optiboard there is an excellent article by Darryl Meister on prism thinning.

**eyeboy** · 05-18-2006, 02:22 PM

Originally Posted by VVizard

Hello, everybody

I would like to ask you what is the common practise in dispensing lenses (especially progressives) regarding thinning (cosmetic) prism. Do you have different policy for plus and minus lenses? If so, why?

Thanks.

there's a really article in the optiboard file directory about prism thinning

**Darryl Meister** · 05-18-2006, 02:25 PM

If you look in the file section of Optiboard there is an excellent article by Darryl Meister on prism thinning.

I actually have it online now at: Prism-Thinning Article.

**optigrrl** · 05-18-2006, 08:32 PM

Thanks for the article - one of my labs decided all of a sudden (on a single-lens replacement job) to discontinue prism thinning unless specially requested. Even if they made the lenses previously and the job referred to the original work order....

Talk about imbalance on the result.

Anyway, the article was a real help.

**eyeboy** · 05-20-2006, 07:47 AM

Originally Posted by optigrrl

Thanks for the article - one of my labs decided all of a sudden (on a single-lens replacement job) to discontinue prism thinning unless specially requested. Even if they made the lenses previously and the job referred to the original work order....

Talk about imbalance on the result.

Anyway, the article was a real help.

Occasionally there is a bug in our surfacing program and no prism thinning is added as a result the lenses always come out thick or waste at the bottom. Perhaps your lab is reducing lens thickness and allowing for the lens wasting at the bottom - however i can only imagine this working in shallow frames????
If we glaze a one lens varifocal we always check the other eyes prism thinning before we surface the job even if we have done the job before.

**optigrrl** · 05-20-2006, 11:51 AM

eyeboy,

That's what I thought. The lab should have double checked the original work order or even the other lens. Instead, they did what they did without question because of a policy change and neglected to inform their customers of this change.

**VVizard** · 05-21-2006, 04:49 AM

Originally Posted by OPTIDONN

... This is only done on plus powered lenses.

Why wouldn't you want to do this on minus lens also? I understand that it will not reduce center thickness of minus lens, but still equi-thining will make better cosmetics... so what are actually the arguments not to do it on minus lens?

**optixbil** · 05-21-2006, 07:35 AM

We prism thin all progressives. Trying to decide where the cut off is becomes a nightmare. If you have a +1.00 OD and -1.00 OS, what do you do? One lens is ordered, the dispenser didn't notice whether the old pair was prism thinned, what do you do? We feel prism thinning them all gives us a better chance of being right. It's not an optical decision, it's a practical decision.

Bill Belanger

**eyeboy** · 05-22-2006, 06:30 AM

Originally Posted by VVizard

Why wouldn't you want to do this on minus lens also? I understand that it will not reduce center thickness of minus lens, but still equi-thining will make better cosmetics... so what are actually the arguments not to do it on minus lens?

The article talked about, earlier in the thread gives diagrams as to why prism thinning, base down, is needed with postive power progressives. it also shows that with a minus PAL lens base up prism can be used to thin the lens!

**OPTIDONN** · 05-22-2006, 11:35 AM

Originally Posted by VVizard

Why wouldn't you want to do this on minus lens also? I understand that it will not reduce center thickness of minus lens, but still equi-thining will make better cosmetics... so what are actually the arguments not to do it on minus lens?

I have done it on both plus and minus lenses. It hardly makes a difference on minus lenses when it comes to thickness.

**eyeboy** · 05-23-2006, 11:37 AM

Originally Posted by OPTIDONN

I have done it on both plus and minus lenses. It hardly makes a difference on minus lenses when it comes to thickness.

Yeah I haven't even bothered on low minus, although I think some high minus 1.74 PAL from seiko had base up.

**OPTIDONN** · 05-23-2006, 04:51 PM

The small amount of prism found in progressive lenses can be a result of the blocking process. When a progressive lens is blocked you put pressure on the distance portion of the lens. This uneven pressure can result in around 1/2 diopter of prism. Shanbaum is a much knows more about this.

**eyeboy** · 05-26-2006, 01:11 PM

Originally Posted by OPTIDONN

The small amount of prism found in progressive lenses can be a result of the blocking process. When a progressive lens is blocked you put pressure on the distance portion of the lens. This uneven pressure can result in around 1/2 diopter of prism. Shanbaum is a much knows more about this.

I've done all sorts of checks on this sort of thing. I don't have much problem with horizontal, but varying amounts of thinning prism depending on who blocks the lens. One guy was adding at least 1 base down-thinning- to every varifocal because he wasn't pressing on the distance portion during blocking! I found that Kodak Precise seem to have a mind of their own no matter who blocks it!!!!