Misinformation 101

**Darryl Meister** · 11-10-2006, 03:26 PM

Not infrequently, I happen upon articles and such that contain statements of questionable accuracy. Now, I'm sure that these authors generally have the best intentions, and may have even done a bit of "homework." I'm not trying to pick on them. But some "myths," "half-truths," and "rules of thumb" in this industry are perpetuated to the point that they now obscure the very facts upon which they were (perhaps loosely) based. And it seems like I see these more and more with some of the new technologies out there today.

So, I'm going to post a few statements, rules of thumb, and/or observations in this thread for debate. These may simply be misleading or equivocal, or perhaps entirely wrong. (For those of you have come across similar examples, feel free to post your own for discussion.) I'm not concerned with the source here, just the validity of the statement. And don't worry if you may have been under the same impression; this is an opportunity to argue the point either way.

The First, and one I see often, is one I ran across again earlier today:

Using 8 degrees of pantoscopic tilt drops the optical center 4 mm from the pupil. (Or some variation of this.)

True of False? Think about what is really being said here...

**HarryChiling** · 11-10-2006, 04:03 PM

Originally Posted by Darryl Meister

True of False? Think about what is really being said here...

Absolutely false!

The proof:

if the visual axis were to be tilted you would get

sin(t)=oc drop/center thickness

sin(8)=oc drop/2.2mm

0.139=oc drop/2.2mm

oc drop=0.139*2.2mm

oc drop=0.306mm

This is just from a trig standpoint, I am sure their are other factors like prism induced that would play a role. I will look into it further at home.

PS - I have got to admit, I have myself spread what I thought to be truths around. I think this forum is a great resource for that. I am reminded of my last post with shanbaum on plano lenses where I ate crow. :)

**shanbaum** · 11-10-2006, 04:52 PM

Originally Posted by HarryChiling

I am sure their are other factors

...like where the pupil is positioned in the frame?

**FVCCHRIS** · 11-10-2006, 04:53 PM

Crow is good if sufficiently cooked and properly presented:bbg:

**HarryChiling** · 11-10-2006, 04:59 PM

Here is an example of what I think is why the rule of thumb seems to be around, and why I think it is inaccurate.

**OPTIDONN** · 11-10-2006, 05:54 PM

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

**HarryChiling** · 11-10-2006, 06:10 PM

Originally Posted by OPTIDONN

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

Donn,

I don't think that is totally correct. I have always had a sort of problem with that concept. If this case were to be correct than all progressives would need to be fit 4-5mm below the center of the pupil. I agree that the OC should drop, but I think what bothers me is the amount 4-5mm in the scheme of things seems like alot.

**OPTIDONN** · 11-10-2006, 06:22 PM

Harry I'm just responding to the question above. Tilting a lens 8 degress will not displace the oc 4mm but the oc will have to be adjusted if there is significant panto. Besides if there is approx 8 to 10 degrees of panto how far would you have to drop the oc in order for the lenses optical axis to pass through the eyes center of rotation.

**Darryl Meister** · 11-10-2006, 07:09 PM

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

I think Donn has done a commendable job of summing this up.

Harry is on the right track as to how the original "rule of thumb" was first developed. The thinking is that the optical axis of the lens should be made to pass through the center of rotation of the eye in order to minimize astigmatism due to lens tilt (and, for a corrected curve lens at least, this is true enough). Using basic trigonometry you can demonstrate that, if the optical center happens to be 4 mm below the pupil at a typical vertex distance, roughly 8 degrees of pantoscopic tilt (2 degrees for each millimeter) will cause the optical axis of the lens to pass through the center of rotation of the eye.

However, to Donn's point, adding 8 degrees of pantoscopic tilt will not cause the optical center to drop 4 mm (in fact, it will effectively raise the optical relative to the line of sight by an amount proportional to the cosine of the angle of tilt).

The next one, and one that is so common we see it posed as a question on OptiBoard regularly:

Aspheric lenses (in general) improve optics and vision in the periphery of a lens.

**HarryChiling** · 11-10-2006, 07:19 PM

Originally Posted by Darryl Meister

If 8 to 10 degrees of pantoscopic tilt are present the oc should be dropped 4-5 mm below the pupil center. Inducing 8 to 10 degress does not automatically drop the oc.

I think Donn has done a commendable job of summing this up.

Harry is on the right track as to how the original "rule of thumb" was first developed. The thinking is that the optical axis of the lens should be made to pass through the center of rotation of the eye in order to minimize astigmatism due to lens tilt (and, for a corrected curve lens at least, this is true enough). Using basic trigonometry you can demonstrate that, if the optical center happens to be 4 mm below the pupil at a typical vertex distance, roughly 8 degrees of pantoscopic tilt (2 degrees for each millimeter) will cause the optical axis of the lens to pass through the center of rotation of the eye.

However, to Donn's point, adding 8 degrees of pantoscopic tilt will not cause the optical center to drop 4 mm (in fact, it will effectively raise the optical relative to the line of sight by an amount proportional to the cosine of the angle of tilt).

The next one, and one that is so common we see it posed as a question on OptiBoard regularly:

Aspheric lenses (in general) improve optics and vision in the periphery of a lens.

False again,

If you are talking about deformed conicoinds than in a certain high plus range yes the statement would be true, however aspherics are used to deviate from best form lenses curves without introducing the off axis errors asscoiated with a deviation from best form.

**Robert Martellaro** · 11-10-2006, 07:28 PM

Put another way (it makes it easier for me to understand it), tilting the lens changes the optical axis of the lens, or a line drawn through the centers of curvature of both lens surfaces. This line, or ray of light, should intersect the center of rotation of the eye, a point near the middle of the eye. If not, then the eye will not look along this line, forcing an oblique angle, potentially introducing oblique astigmatism and power error.

So the correct statement should be something like- the optical center should be moved below the pupil .5mm per one degree of panto.

**Darryl Meister** · 11-10-2006, 07:39 PM

So the correct statement should be something like- the optical center should be moved below the pupil .5mm per one degree of panto.

Yep. The thing to remember is that tilting the lens doesn't automatically move the optical center 0.5 mm per degree though.

If you are talking about deformed conicoinds than in a certain high plus range yes the statement would be true, however aspherics are used to deviate from best form lenses curves without introducing the off axis errors asscoiated with a deviation from best form.

Very good answer. Below around +7.00 D, aspheric surfaces do little more than free you from the constraints of "best form" or "corrected curve" Base curves, allowing lens designers to use flatter (or even steeper) curves, while maintaining the optical performance of traditional best form designs.

The next one, and one that I have run across several times over the past 15 or 16 years:

Antireflection coatings reduce (if not eliminate) the effects of chromatic aberration in high-index and polycarbonate lens materials.

**lensgrinder** · 11-10-2006, 08:32 PM

Originally Posted by Darryl Meister

Antireflection coatings reduce (if not eliminate) the effects of chromatic aberration in high-index and polycarbonate lens materials.

Well I would have to say False. Chromatic abberation is the prism amount divided by the abbe' number. So putting an AR coating does not help. By the way this is an awesome thread.

**HarryChiling** · 11-10-2006, 09:58 PM

Originally Posted by lensgrinder

By the way this is an awesome thread

Sweet thread, lots of good misconceptions being addressed.

Originally Posted by Darry Meister

Antireflection coatings reduce (if not eliminate) the effects of chromatic aberration in high-index and polycarbonate lens materials.

Absolutely false again,

chromatic aberration is due to the propertys of the material refracting the various wavelengths of light at different degrees. Like lensgrinder said it's the prismatic effect divided by the abbe number.

I would venture to say that it *might* be possible to deposit coatings with abbes that might help offset the base materials chromatic aberration, just a thought.

**Darryl Meister** · 11-11-2006, 11:31 AM

Well I would have to say False. Chromatic abberation is the prism amount divided by the abbe' number. So putting an AR coating does not help.

That pretty much sums it up.

The next one I've been running across a lot lately:

"Free-form" or "digital" lenses are back surface progressives.

**OPTIDONN** · 11-11-2006, 01:02 PM

free form is a manufacturing process not a style of lens. This is fun!

**lensgrinder** · 11-11-2006, 06:51 PM

Originally Posted by Darryl Meister

The next one I've been running across a lot lately:

"Free-form" or "digital" lenses are back surface progressives.

I would have to say false again. Progressive lenses obtain there add power from increasing curvature on the front surface. Also, "Free Form" could be used for any lens, not just progressives.

**OPTIDONN** · 11-11-2006, 08:01 PM

Originally Posted by lensgrinder

I would have to say false again. Progressive lenses obtain there add power from increasing curvature on the front surface. Also, "Free Form" could be used for any lens, not just progressives.

Most of the free form generated lenses today have thier curves on the back of the lens our Seiko Succeed is 100% internal which means that the front is spherical while all the curves are on the back. It's pretty neat!:bbg:

**optigrrl** · 11-12-2006, 10:55 PM

I agree with donn and lensgrinder, Free-form is a grinding process and whether it is ground on the front, back or side it is a description of the process of creating the rx from a blank and not a semi-finished or cast-molded form.

**Fezz** · 11-13-2006, 07:32 AM

I always like to see or hear someone say how a anti-reflective coating eliminates "glare".

**EyeFitWell** · 11-13-2006, 09:01 AM

Technically, isn't glare just ambient light which has been reflected off a surface? So, we talk alot about glare and polarized lenses removing glare from the patient's view, but doesn't AR cut "glare" off your lens itself?
I agree it's misleading, and I've heard people come and complain that their AR doesn't help with the bright sun outside, but I believe it's technically correct...

**shanbaum** · 11-13-2006, 10:16 AM

Originally Posted by EyeFitWell

Technically, isn't glare just ambient light which has been reflected off a surface? So, we talk alot about glare and polarized lenses removing glare from the patient's view, but doesn't AR cut "glare" off your lens itself?

I don't know if there's a really good definition of glare in an authoritative text or not. My understanding of the term is that it describes an amount of light that exceeds a viewer's ability to accept comfortably. That can occur when the amount of light is absolutely excessive (which means, the viewer's pupil can't stop down any further), or when the contrast range of the scene exceeds the viewer's contrast range (e.g., oncoming headlights at night, when the pupil is dilated).

I think the lexical confusion arises because reflected sunlight is a common source of glare, and polarizing filters can often selectively absorb that.

I suppose it's possible to have a surface reflection on an uncoated lens so bright that it passes the threshold of discomfort, but the source might vaporize the wearer (problem solved).

**EyeFitWell** · 11-13-2006, 02:35 PM

I have soo many patients in a fairly weak Rx who say they've been told they CANNOT do anything without a full rim. (I'm talking about -3's and 4's...here I am wearing my -6 in a completely rimess frame!) It can be done! I don't see why people say you CAN'T...such a harsh word.

**Alan W** · 11-13-2006, 05:01 PM

If you step back and look at lens edges it's possible to treat that as a unique cosmetic enhancement. By that I mean a higher powered lens, particularly a higher minus, with a flat edge highly polished and engraved or faceted can actually become a "work of art" that appears very much "intentional". I've made hundreds of such eyewear pieces selling them from samples that have been designed and crafted to look intentional. They can even be tinted on the edge as well as powder coated solid and in patterns. I believe there will be a resurgence of that technique which was very popular in the 80's. Some of us who have photos of Elton John's eyewear are examples crafted by Hans Feibig (of Optique Boutique) in Hollywood along with a few others.