2 drop vs 4 drop progressive lenses

**Opticalman1** · 04-14-2009, 10:54 AM

Hello all,

We have a doctor that will only use 2 drop progressive lenses and not any of the more popular ones that HOYA and Essilor have that are 4 drop.
How much difference is there between the two designs and if not how can I explain it to him?

Thanks!

**Fezz** · 04-14-2009, 11:14 AM

What is the Doc's reasoning?

Is it about optics? Cost? Loyalty to a rep or manufacturer?

**Opticalman1** · 04-14-2009, 11:24 AM

Thanks for the reply,

He thinks the optic on the two drop are better and he has more adapts with these than 4 drop.

Thanks

**obxeyeguy** · 04-14-2009, 12:06 PM

Color me slow here, but what the heck is a 2- drop progressive? The only "drop" lens I remember was a welsh. Have never heard the designs referred to as drops.

**Opticalman1** · 04-14-2009, 12:27 PM

Thanks for the reply,

The drop is the difference between the height of the fitting cross and the height of the engraved hidden circles for placing the verification masks on the lens.

Example the VX Comfort has a difference of 4mm and a Sola VIP has a difference of 2mm.

Thanks

**DragonLensmanWV** · 04-14-2009, 01:08 PM

That's all just a matter of manufacturer's preferences. To only use that as a sole criteria for using a lens is very short-sighted and mis-informed. If only "2-drop" progressives are successful, then why are they in the minority? It's one thing to use a lens because you've had good success with it, but to dismiss out of hand over half the designs is a poor decision made on zero facts that robs his patients of many excellent designs. I think it's because he has made his own decision without having any facts. Time for him to start catching up with the times.
As a note, we've had around 98% success with all types of progressives (except the "2-drop" SolaMax), even the old ones you had to have a 24mm minimum seg height. And at that time there were none that had a "2-drop".

**obxeyeguy** · 04-14-2009, 02:24 PM

Originally Posted by Opticalman1

Thanks for the reply,

The drop is the difference between the height of the fitting cross and the height of the engraved hidden circles for placing the verification masks on the lens.

Example the VX Comfort has a difference of 4mm and a Sola VIP has a difference of 2mm.

Thanks

Thanks! I know the diff between the 2 and 4 lenses, just never heard it used in that context before.

**CME4SPECS** · 04-14-2009, 03:09 PM

Originally Posted by Opticalman1

Thanks for the reply,

The drop is the difference between the height of the fitting cross and the height of the engraved hidden circles for placing the verification masks on the lens.

Example the VX Comfort has a difference of 4mm and a Sola VIP has a difference of 2mm.

Thanks

I'm pretty certain that the only difference is as you describe it. There is no difference in fitting or performance of these 2 different types of markings. Just different markings.

**drk** · 04-14-2009, 05:47 PM

I was under the impression that the "2 drop" meant 2 mm from the fitting cross to the PRP, and the "4 drop" meant 4 mm from the cross to the PRP.

**Robert Martellaro** · 04-14-2009, 06:43 PM

Originally Posted by Opticalman1

The drop is the difference between the height of the fitting cross and the height of the engraved hidden circles for placing the verification masks on the lens.

Right. Those circles are on the datum or 180 line.

Originally Posted by drk

I was under the impression that the "2 drop" meant 2 mm from the fitting cross to the PRP, and the "4 drop" meant 4 mm from the cross to the PRP.

Yes, the PRP (prism reference point) is on the 180 line 17mm equidistant from the circles/symbols.

Originally Posted by CME4SPECS

There is no difference in fitting or performance of these 2 different types of markings. Just different markings.

Right. I ignore it except for anisometropes. It should be noted that the drop can be anywhere from 0 (most of the Seiko/Pentax) to 6mm for some Zeiss PALs.

**drk** · 04-15-2009, 11:59 AM

Well, I'm a "3-dropper" on my SV lenses for my standard ~6 degree panto--one mm drop OC per 2 degrees panto.

So, on 2-drop designs, do they require 4 degrees panto and on 4-drop an 8 degree tilt? Seems so, but I haven't thought of that...

And to be clear: the appearance of the first +0.25 of add is not related to this issue, right?

**Robert Martellaro** · 04-15-2009, 01:02 PM

Originally Posted by drk

Well, I'm a "3-dropper" on my SV lenses for my standard ~6 degree panto--one mm drop OC per 2 degrees panto.

The Gradal Brevity has a 3mm drop!

So, on 2-drop designs, do they require 4 degrees panto and on 4-drop an 8 degree tilt? Seems so, but I haven't thought of that...

By the book, yes. A little extra panto will probably do more good (zone width at near) then harm (sph and cyl error) unless the powers are high. Not an issue if the design is compensated for position of wear.

And to be clear: the appearance of the first +0.25 of add is not related to this issue, right?

Yes. Where the change starts, the rate of change, and corridor length are all independant of the drop. There are short corridor 4mm drops, and long corridors with 2mm drops.

**chip anderson** · 04-15-2009, 02:13 PM

I was just looking through My Progressive Lens Identifier 2009 edition and trying to decern things I don't know or have forgotten. Found that some progressives are 2mm, some 3 mm, some 4 mm, some 6 mm, and some 0 mm. As I was wondering just what significance this had on my doings, the Hoya Rep came in and I axed him (he's Cajun so, I can axe him.)
He referred to these indications as "drops".
When I axed him what significance does this have in fitting and measuring for the lenses, his reply was none except, you need to go that high above the fitting cross to check power.
No difference in measureing fitting height or how high the patient must look to obtain distance zone, or how low to achieve near.

Chip

**Darryl Meister** · 04-15-2009, 02:47 PM

How much difference is there between the two designs and if not how can I explain it to him?

As DragonLensman stated, the "drop" of the lens simply reflects the manufacturer's preference. It generally has very little influence on the optical performance of the lens.

The prism reference point simply represents the intended location of the optical center in the absence of prescribed prism or prism-thinning. So, a "4 drop" lens would traditionally have the optical center located 4 mm below the fitting cross, whereas a "2 drop" lens would have the optical center located 2 mm below the fitting cross.

Of course, this is somewhat arbitrary. And there are also "8 drop" and "6 drop" and even "0 drop" lenses, as Chip noted. There are certain mechanical advantages to the use of certain "drop" distances, in terms of maximizing lens cut-out, minimizing blocking waves during the surfacing process, and so on, but the differences are generally small.

Although one could make an optical argument regarding the placement of the optical centers below the lines of sight, this becomes moot for modern progressive lenses, since the majority are surfaced with at least some degree of prism-thinning. Consequently, regardless of the initial "drop" of the lens design, the actual location of the optical center is controlled by the surfacing software.

**drk** · 04-16-2009, 03:39 PM

Yeah, I considered prism thinning's influence on the optical centers, but my brain exploded all over my keyboard.

Prism thinning will move the OCs to an altogether different location than the PRP mark on the lens, rendering them a mere pretty freckle.

What, then, of the imperative of the optical axis hitting the sighting intersect (center of rotation) of the eye? Is it null for progressives with prism thinning?