an inquiry about pantoscopic tilt

**safeir** · 02-08-2011, 08:23 AM

dear colleagues
i am a beginner in the field of optics , and am wondering about the pantoscopic tilt . should i make the compensation (1 mm down-decentration for every 1 degree of tilt) for all kind of lenses , single vision & multifocals , aspheric & spherical lenses , ...... etc ?
or this procedure should be confined on this type of lenses not that ?
kindly give me a short answer, then provide links to more details sources if you want .
thanks in advance

**drk** · 02-08-2011, 10:37 AM

1 mm down per 2 mm panto.

All types of SV, only.

**jherman** · 02-08-2011, 11:18 AM

there are occasions with moderate rx's, in flat-tops, where tilt is warranted.
trust me;)

**Robert Martellaro** · 02-08-2011, 02:04 PM

I'm using about eight to ten degrees of panto tilt for most general purpose eyeglasses.

Martin's formula for tilt applies to multifocal as well as SV, at least for the distance portion.

Be aware that vertical prism imbalance will be induced if you lower the OC's on Rxs that have unequal powers on the vertical meridian.

**NeGlassesGirl27** · 02-08-2011, 02:06 PM

A poster in Egypt and using the internet? I thought it got shut off? (Off subject..I know)

**drk** · 02-08-2011, 07:27 PM

I used to specify level MRPs on my segmented MFs until the lab owner called me and said that they have to surface the job if I do specify.

Being a nice guy, I desisted.

IIRC, the OC is always, what, 4 mm above the seg line in finished lenses?

Maybe I got that whole thing wrong way back when.

**HarryChiling** · 02-08-2011, 08:23 PM

In the case of multifocals you want to lower the DRP not the seg line. In most cases if you don't specify the DRP or OC height on a segmented multifocal you will get the labs default, I have heard 5mm, 4mm, but in most cases spliting the B measure is the method I prefer. For children in multifocals where the doctors prescription specifies to split the pupil I ask for the OC to split the B measure to reduce thickness differences.

**drk** · 02-08-2011, 10:33 PM

Here's my method:
In order to keep lens measurements consistent, I always dot the pupil center of every lens and take split pds.

For segmented BFs for standard use, I drop the seg line 8mm below pupil center. +/-2mm depending on the circumstance. On sunwear I drop the seg 12 mm.

So, with a standard 8mm drop, if the lab puts the MRP 4 mm above the seg line, I'm right on for the average 8 degree panto.

You're right, though, on the sunglass segs or unusually high segs (or even anisos) I'd better specify.

I'd better consult my lab again.

**shanbaum** · 02-09-2011, 08:37 AM

I think you mean PRP (which may be coincident with the DRP).

**HarryChiling** · 02-09-2011, 10:43 AM

Originally Posted by shanbaum

I think you mean PRP (which may be coincident with the DRP).

Thanks, I was assuming they were coincident but PRP would be a better description of the point in reference.

**Robert Martellaro** · 02-09-2011, 11:00 AM

There's also the design reference point that needs consideration- if the lens is aspheric, lowering the seg on golfing glasses will most likely take this point out of its optimum position. The only remedy is to use a spherical design.

Most segmented multifocals have the OC about 3mm above the top of the seg for bifocals, and about 1mm above the seg for trifocals.

**HarryChiling** · 02-09-2011, 11:23 AM

Originally Posted by Robert Martellaro

There's also the design reference point that needs consideration- if the lens is aspheric.

I think they call that the LRP or layout reference point. Your dead on with the average segments being above the seg line about 3mm for MF and 1mm for TF. In most cases the default values are fine.

**drk** · 02-09-2011, 02:10 PM

So 3 mm above seg line...

BF seg line in my office set standard at 8mm below pupil...

Then the MRP (or whatever we call it) is 5 mm below pupil center which is correct for 10 degrees panto. Good enough for government work.

With TF, I set them 4 mm below pupil center, then the OC's 3 mm below pupil center. Again, about right for 6mm panto.

As you can tell, since very few people even think about this and there's no major optical crisis been going on, it must work out on the default values almost every time. I like defaults.

**Barry Santini** · 02-10-2011, 07:35 AM

Originally Posted by drk

So 3 mm above seg line...

BF seg line in my office set standard at 8mm below pupil...

Then the MRP (or whatever we call it) is 5 mm below pupil center which is correct for 10 degrees panto. Good enough for government work.

With TF, I set them 4 mm below pupil center, then the OC's 3 mm below pupil center. Again, about right for 6mm panto.

As you can tell, since very few people even think about this and there's no major optical crisis been going on, it must work out on the default values almost every
time. I like defaults.

All of the above doesnt matter if you're not using the proper curve for the index selected, as well as the vertex and other compensations if the frame plane is other than flat.

Its good to reaquaint ourselves with the principles of best form fitting

B

**drk** · 02-10-2011, 02:15 PM

Originally Posted by Barry Santini

All of the above doesnt matter if you're not using the proper curve for the index selected, as well as the vertex and other compensations if the frame plane is other than flat.

Its good to reaquaint ourselves with the principles of best form fitting

B

Re-acquaint me, please.

I don't specify base curves, my lab does. I don't have the knowledge or the software.

How does vertex factor in on the level MRP? I mean, extreme cases maybe...

How does face-form factor in on level MRP?

**Barry Santini** · 02-11-2011, 10:26 AM

Originally Posted by drk

Re-acquaint me, please.

I don't specify base curves, my lab does. I don't have the knowledge or the software.

How does vertex factor in on the level MRP? I mean, extreme cases maybe...

How does face-form factor in on level MRP?

The higher the index, the flatter the base curve (compared to crown glass/plastic) to adhere to best form criteria. There are rules of thumb, but remember choice of base often has to do with *available* bases, which means a regular, unintended departure from optimal Best Form fitting. SV FF overcomes this entirely. It's part of its WOW, and what makes even spherical-only fits deliver the WOW.

Yes, close vertex (8mm) or long 19+ requires a new trig calculation. But then again, so the base curve might change as well under these POW values.

Face form? Well, unfortunately the effects of face form are not *yoked*, so any reasonable degree of face form makes off axis performance degrade significantly for *binocular* viewing. Say, 5-8 degrees or more, depending onn the strength of the Rx. Although it doesn't influence vertical MRP, clearly horizontal is affected.

And propeller is worse, which is why this mal-adjustment drives people in more than any other.

B

**drk** · 02-11-2011, 03:06 PM

Is "propellar" non-coplanarity of the lenses about the X-axis?

**Darryl Meister** · 02-11-2011, 09:23 PM

Is "propellar" non-coplanarity of the lenses about the X-axis?

Yes. Also called "X-ing."

you will get the labs default, I have heard 5mm, 4mm, but in most cases spliting the B measure is

This is probably a setting that you can specify in the more sophisticated software packages. Generally, keeping the optical center at the datum (180) line is preferable in the surfacing software, which will minimize thickness and weight. Although, in some cases depending upon the software, there may be a limit of 3 mm or more from the segment ledge, so that the ledge doesn't interfere with centering the optical center in front of a focimeter aperture during inspection.

Best regards,
Darryl