GT2 Questions (for Darryl??)

[Barry Santini]

I have a few questions about parametric measurements for GT2 (and progressives in general):

1. Should DV mono PD's be used, or should Mono NV PD's be used? If Mono NV PD's, should the dispenser keep the specific inset used by Zeiss for a set of GT2s, and center the NV to match the actual insetted-NV, or just go with the DV PD's. I've found that essilor-pupilometer-derived mono NV PD measurements are often not *symetrical*(by this i mean that the NV PD's are not equal to the DV PD's, less 2.5mm each) with the actual DV taken. Please note that I do take the measurements a few times to confirm their repeatibility.

2. Is the layout "Comp" (*my* word for the mm distance between the fitting cross and the MRP) still 6mm for a Zeiss progressive? (and why is it 6mm anyway??? I think Zeiss should consider moving to the more common 4mm, just to eliminate fabrication and checkout errors when using "circles" as reference point during edging and subsequent verifications)

3. What are the *default* values Zeiss assigns for POW (position-of-wear: phoropter/focimeter parameter compensation; VD, PANTO, FACE FORM, nearpoint vergence) for GT2?

4. Is GT2's rear surface "free-form"? (not evident from the white paper)

Thanks in advance for your responses. BTW, I'm not trying to ask anything that is *proprietary*

Barry Santini, ABOM

[drk]

How could the back side be "freeform" or "digitally surfaced?" If so, they'd cost a fortune. As is, they go for a resonable penny.

[For-Life]

take a mono distance pd, mono near pd, and mono fitting height on every pair.

[OPTIDONN]

How could the back side be "freeform" or "digitally surfaced?" If so, they'd cost a fortune. As is, they go for a resonable penny.

Plenty of free form lenses are back surface. For example Sieko's Succeed, Pentax's Perfas, and Kodak's Unique, just to name a few. Not all free form lenses cost an arm and a leg;)

[Darryl Meister]

1. We recommend the use of mono distance PDs, though there are obviously pros and cons to either approach. We generally don't recommend using the near PDs, since the design employs a variable inset, which does a good job of maximizing binocularity at near. Besides, since the inset varies for every Base curve and Add power combination, you would need to look up the individual near inset from something like 72 possible Base and Add combinations in order to use the NV method. The inset ranges from 2.0 mm, in the low Add/low Base combinations, to 4.5 mm, in the high Add/high Base combinations.

2. We agree with you completely, and the new GT2 does indeed use a distance of 4 mm from the prism reference point to the fitting cross, and a distance of 8 mm from the prism reference point to the distance reference point. These values are quickly becoming an industry de facto standard. I've never bothered to ask the Zeiss lens designers, but the use of 6 mm for earlier Zeiss products probably relates to blocking stability, vertical prism control, the coordinate system of their lens design software, and that sort of stuff.

3. I don't recall what the position of wear values are off the top of my head, but I have them around here somewhere. I'll have to hunt them down. My guess is that a pantoscopic angle of around 9 or 10 degrees was used, along with a stop distance of around 27 mm and a near fixation point of around 38 cm (for Adds less than +2.50 D; 1/Add for Adds above that). The actual values may differ slightly. Zeiss lens designers frequently base these position of wear values on the data they routinely see from Gradal Individual orders and RVT measurements. I should also note that Zeiss lens designers typically vary these optimization parameters some depending on the specific Base curve and Add power combination.

4. GT2 by ZEISS is a semi-finished lens, not a free-form lens, so it uses a traditional front surface progressive lens design. Gradal Individual is our free-form product by ZEISS, which is either a front-surface or a back-surface progressive depending upon the prescription range. That said, ZEISS does cut all of their progressive molds directly using a free-form generator (as opposed to "slumping"), which some manufacturers are now marketing as "free-form progressive technology" (e.g., Shamir's semi-finished Creation lens).

We should have the white paper for GT2 out in two or three weeks, which will get into more technical detail. Feel free to touch base with me in a week or two about it.

[Barry Santini]

Thanks for your reply, Darryl.

And yes... I do take mono DV PDs, Mono NV's and Mono Heights on all clienst

barry

[Clive Noble]

Dear Darryl, I don't want you to waste too much time on a doctorate to my question, but I see the GT2 can be fitted as low as 18mm (or was it 17?)
The fitting cross is at 4mm not the traditional Gradal 3, HS and Top: 6mm.

These parameters seem to be similar to the Brevity.

Apart from the 'Z' markings that don't appear on the USA Brevity, is there much difference between these two designs?

Best regards, Clive

[Darryl Meister]

GT2 is actually more similar to Gradal Top (the "GT" in "GT2"), but with a significantly shorter corridor length and several other optical refinements. GT2 also has a minimum fitting height of 17 mm.

Brevity has a minimum fitting height of 16 mm, and would still be the "small frame" progressive of choice from the ZEISS family, though there are obviously fewer jobs that fall into this category since GT2 offers a relatively low minimum fitting height. Brevity is very different from both Gradal Top and GT2. In addition to a very short corridor length, the Brevity design is a softer in the distance periphery compared to the other two, which have very wide, clear distance zones.

[Clive Noble]

I've just come home after edging and fitting my new GT2s.

I have a dramatic change in the Right eye due to a fast moving cataract and the Left has also changed slightly.

I have been wearing Gradal Top for many years and in fact put these new lenses into my last frame in place of the Top which had been fitted at 22 high.

As the GT2 can be fitted asa low as 17, I gave myself the luxury of dropping the fitting cross to 21 to give a deeper distance portion.

The drive home from the lab was great.

These lenses are just a delight, the D/V horizontally is like S/V, clear and undistorted to the edges of both lenses, I don't see the N/V section being any wider than the GTop, which was good anyway, and the intermediate is crystal clear, but the head and eyes have to be more focussed to get the right position.

I think I'm going to like these lenses..........

[Fezz]

Clive,
Thanks for the review. Keep us posted. Have you tried on any patients yet?


Fezz
:cheers:

[Clive Noble]

I'm now 4 days into my GT2 Transitions.

The Distance is just amazing, as I said in my last post, just like S/V edge to edge.

The near is as good as the old Gradal Top, but I'm having slight computer problems with the intermediate, I just can't find a really comfortable position. I've moved the screen and the chair, I seem to think that the old Top had a better field in the 90 degree angle.

Don't get me wrong, I have got used to this situation and there's no problem with working on a computer for 20 minutes or so, but for longer periods I'll have to make up a simple pair of Int. S/V

[Darryl Meister]

The Distance is just amazing, as I said in my last post, just like S/V edge to edge. The near is as good as the old Gradal Top, but I'm having slight computer problems with the intermediate, I just can't find a really comfortable position.

Honestly, it sounds as though the lenses may be fitting a little low. The distance zone is excellent, but it really isn't any better than Gradal Top's (which was equally expansive), and the corridor length is around 2 mm shorter than Gradal Top's (which should make the near zone significantly easier to reach).

Also, has your Add increased? This could help explain your reduced intermediate utility, particularly if the lenses are 1 or 2 mm too low.

[Clive Noble]

Yes Darryl, you're correct, I did fit them about 1mm below pupil centre, the add remains the same (+3.00)

Interesting to note that the corridor is only 2mm less than 'TOP'
I hear what you say, maybe it's psycological but the peripheral distance is certainly better on the GT2s.

So now I'm confused, you say the distance is the same, I claim that the near is the same, we know that the GT2 is 2mm more 'condensed' than the Top, so what is the reason for introducing a more expensive product when there's already the Brevity on the market...... or is the reason a facelift on a successful model that has been around for many years as the motor industry does.

One thought, nothing to do with 'our' topic, is that over the years we were told only to fit HS and TOP @ 24mm.

I have to admit, I've been naughty and have been fitting them even as low as 22mm without any problems, then I hear colleagues tell me... "No, you can fit them as low as 18" so over the last year, I have done the occasional 19 and surprisingly, no complaints.

In retrospect, I feel we lost out on a lot of, what could have been, Zeiss sales in favour of other brands.

[Fezz]

Clive,
I have fit the Top as low as 17.5 high. I know that a lot of the reading zone is cut off, but suprisingly, very few problems. We have fit boatloads at 18 to 20 with great results. I tried a few Brevitys, but had mixed patient response. Funny how with a little patient education and proper dispensing and adjustment can make all the difference in patient satisfication.

[Darryl Meister]

The lens designers at Zeiss have always been "sticklers" for maintaining optical performance above all else, and were reluctant to change their minimum fitting height recommendations as aggressively as other manufacturers.

[jofelk]

The lens designers at Zeiss have always been "sticklers" for maintaining optical performance above all else, and were reluctant to change their minimum fitting height recommendations as aggressively as other manufacturers.

I like the approach Zeiss takes. Telling opticians seg heights are critical and then changing the rules to suite changing fads in frame design causes alot of confusion.