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**drk** · 04-27-2004, 09:43 AM

I'm trying to understand how progressive designs vary by prescription!

INSET
If someone is high minus, they get BI prism when they converge to read, and BO for high plus. Are insets variable on some newer, individualized designs to counteract this, e.g. less inset for high minus?

Are conventional progressives mostly 2mm in, regardless, since a near pd usually= distance pd minus 3-4 mm? Or, does a particular semi-finished progressive blank have an inset that is maximized for the basecurve, knowing that on a +8 base, you are going to end up with +2.00 to +4.00 (whatever)?

FIELD OF VIEW
If someone has higher minus, the field of view through a progressive near zone should be larger, due to minification of the object. Higher plus would have a smaller field of view. Do designers try to keep the near zone constant by moving distortion into the distance with higher plus lenses, or by making the periphery harder, or both, or neither? And can designers, therefore, make a softer periphery for a minus Rx?

ADD POWER
If the add is lower, there is more room to play with for the viewing zones, and less with higher adds. Which designs value distance area the most, and keep it constant, which value near the most, etc.? Do most designers try to keep the proportion of distance to near the same across add powers?

PRISMATIC THINNING
And another thing, what is prismatic thinning? 1-2 pd yoked BU prism to decrease thickness in the add portion? What's that all about? Does that vary with optical labs? Materials? Add powers?

**edKENdance** · 04-27-2004, 12:20 PM

Make us smarter. Great questions!

**Robert Martellaro** · 04-27-2004, 01:42 PM

I'm trying to understand how progressive designs vary by prescription!

INSET
If someone is high minus, they get BI prism when they converge to read, and BO for high plus. Are insets variable on some newer, individualized designs to counteract this, e.g. less inset for high minus?

No. You'll need some type of fused multifocal that has sufficient width to work with i.e. ST35. You could Rx BI prism for the plus lenses.

Are conventional progressives mostly 2mm in, regardless, since a near pd usually= distance pd minus 3-4 mm? Or, does a particular semi-finished progressive blank have an inset that is maximized for the basecurve, knowing that on a +8 base, you are going to end up with +2.00 to +4.00 (whatever)?

The latter.

FIELD OF VIEW
If someone has higher minus, the field of view through a progressive near zone should be larger, due to minification of the object. Higher plus would have a smaller field of view. Do designers try to keep the near zone constant by moving distortion into the distance with higher plus lenses, or by making the periphery harder, or both, or neither? And can designers, therefore, make a softer periphery for a minus Rx?

Some manufactuers do. I would guess that the method varies by manufactuer.

ADD POWER
If the add is lower, there is more room to play with for the viewing zones, and less with higher adds. Which designs value distance area the most, and keep it constant, which value near the most, etc.? Do most designers try to keep the proportion of distance to near the same across add powers?

In a 2.25 add I found that the Life 2 is as clear off axis on the 180 as any lens that I've tried. Might be important if your a pilot, truck driver, etc.

PRISMATIC THINNING
And another thing, what is prismatic thinning? 1-2 pd yoked BU prism to decrease thickness in the add portion? What's that all about? Does that vary with optical labs? Materials? Add powers?

BD on plus powers to reduce superior lens thickness.

Robert

**drk** · 04-27-2004, 02:14 PM

Thanks, Robert.
Why are plus progressives thicker at the top? It seems that they would be thicker at the bottom, since there's more plus there.

**CME4SPECS** · 04-27-2004, 02:27 PM

Just like Execs the curvature is greater at the bottom, so the lens will be thinner at the bottom. By grinding the prism you balance the thickness.

**Robert Martellaro** · 04-27-2004, 03:52 PM

Drk,

Vertical decentration. The OC is usually above center inducing BU prism (on lenses with plus power at 90). Convert some of your PAL seg heights to above/below center and you should find that a range of 3mm to 6mm above center is typical. Even with the OC 2-4mm below the fitting cross there is still some BU prism.

Back when big frames were popular the B measurement was 45-50mm with the fitting cross 6-10mm above center. Decenter a +3.00 up 10mm and we are looking at a pretty thick lens, especially with a lens this big. Not much of a problem nowadays with frames more sensibly sized.

Robert

**JRS** · 04-27-2004, 06:02 PM

I can't speak for the other companies, but both Panamic & Ovation lenses have variable insets. Less inset on minus, more on plus powers. It also varies (some) by add power - within the power ranges. Insets range from (about) 2.0 to 5.0mm per lens.

As for Comfort lenses, they are reasonably static, except for the higher adds throughout the power ranges. With the Panamic & Ovation lenses being of newer design, I suppose this is to be expected. However, given the 'wearability' of Comfort, I'm not to sure that a varying inset is a major part of a patient's comfort (spin on words there - lol) or discomfort when dropping down.