"Distortionless" Digitals....Aberrated!

[casting guru]

So the other day, I was trying to re-order a pair of uncuts from an [un-named] wholesale lab because the first pair they sent me were abberrated in the distance. The customer service rep literally says to me, in an argumentative tone, "aberrated? What does that even MEAN? What do you MEAN by that?"

So I was placed on hold, and then the lab manager picked up and asked me, "So what are you using to check out this digital free-form lens?"

Me: "Umm... a lensometer..."

Lab Manager: "YEAH, see, you can't put these digitals on a lensometer...they're all going to appear distorted. They just can't be checked out with a lensometer."

Really? A "distortionless" lens will always look distorted on a lensometer!!!??? I just can't hardly believe that a "lab manager" would say that out-loud!

[chip anderson]

I remember touring a Ciba facility in Atlanta. We thought it was the plant but later learned it was a "research" facility. At one point we met three of the OD researchers. I asked a question about abberration and all three asked: "What's that?"
Just before I left the room I noticed a sign referring to abberation and to look out for it.

For the record all aspheric progressives have abberation, that's how they blend the add power to distance. However many "consultants" will use other terms for it, like sand, cylinder, etc. But it's abberation.


Chip

[Robert Martellaro]

Maybe he meant that you can not measure lens aberrations with a standard lensometer, which is true (edit- except for power error and oblique astigmatism with SV lenses and a modified lens stop). You can observe waves and warpage, and measure for prescibed prism, sphere, cylinder, axis, unwanted prism and positional tolerances, although you need to make sure that the lens is or isn't compensated for position of wear. It should come with a compensated Rx if it is.

Are these PALs? Are you measuring the distance power at the distance reference point? In most cases it's best to use the mask provided by the manufacturer- measure the distance power at the open circle (above the fitting point/distance visual point). You may need to use an auxiliary prism to center the target.

[Wes]

The aberrations radial (oblique) astigmatism and off-axis power error can be measured with a standard lensometer. See ANSI 1964 and 1972.

[Robert Martellaro]

The aberrations radial (oblique) astigmatism and off-axis power error can be measured with a standard lensometer. See ANSI 1964 and 1972.

I didn't know that. You'll need to at least ray trace and/or incorporate a lens mapper for anything more than a simple spherical surface, right?

[casting guru]

(aberrations were in the distance, visible with the naked eye, but also with the lensometer... just very simple... hold it up to the light, "wow, the distance sucks on this pair!")

Really, my amazement was more in the fact that he was saying out loud, all "distortion-less" lenses will look distorted on the lensometer... but the WAY he said it was more like... "don't you DARE question my fancy-schmancy in-house free-form (typical IOT, small cribbed uncut)... if they look aberrated it's because your EYES or lensometer are aberrated, and I know this b/c I'm the king of free-form. Now go away you silly minion and stop bothering me!"

Needless to say, I'm not even getting the replacement pair from the. Who cares about the credit on the job.

[Wes]

I didn't know that. You'll need to at least ray trace and/or incorporate a lens mapper for anything more than a simple spherical surface, right?

Possibly, it depends on what you're looking to do. POW compensation is done to adjust for the aberrations induced by looking through the lens off-axis. The prescribed method for measuring these aberrations has been around for far longer than POW compensation.
I'll post a diagram and some more info when I get home.

[Wes]

Also, progressives will have their traditional distortion. I was referring to spherical surfaces, but I don't see why the technique wouldn't work on aspherics, progressives aside.

[Wes]

"Oblique astigmatism has long been known to be troublesome to the wearer, and as such, was addressed in the original 1964 ANSI Z80.1 and in the subsequent 1972 standards along with tolerance tables and prescribed methods of measuring for oblique astigmatism.12 The 1964 standards recommend testing the principal meridians in a lensometer using a vertex sphere of radius 27 mm at points both horizontally and vertically 30 degrees away from the center of rotation with the back vertex of the optic center to coincide with the surface of the 27mm radius vertex sphere. (See figure 1, taken from ANSI Z80.1, 1972.) The 1972 standards changed the specification from 27 mm to a range between 24 and 33 mm to be specified by the manufacturer. This creates the proper oblique angle to the lens surface for measurement of oblique astigmatism."

[Robert Martellaro]

Thanks Wes. Here's a link to z80 1964.

http://www.opticalheritagemuseum.org...Z80_1-1964.pdf

[Wes]

Thanks, I have a pdf of that too. An extraordinary contributor here shared a copy with me when I was writing my master paper. Funny how the latest standards don't address the higher order aberrations.

[PhiTrace]

Those ANSI standards are tighter than todays standards, one of the major contributors to the lowering of the standards has been the OLA. Many of the same lab memebers of the OLA are now also producing FF lenses. If they were not concerned with the quality of a PAL surface then, it's hard for me to believe that many of these labs would be that concerned now. I have a few labs that I know I can trusts with great results and I have actually seen lenses where the same Rx ordered with all the same variables have produced very different end results from other labs.

I like to think of FF products and labs that produce them as being in the wild west stage of the game.

[rdcoach5]

My local lab digitally surfaces all of their lenses and in the lensometer they check noticeably sharper. You do have to use an auxillary prism to center the target as stated by Robert but is is possible to get an abberrated lens if ground too thin in the center.This can occur on high minus especially.

[Barry Santini]

Possibly, it depends on what you're looking to do. POW compensation is done to adjust for the aberrations induced by looking through the lens off-axis. The prescribed method for measuring these aberrations has been around for far longer than POW compensation.
I'll post a diagram and some more info when I get home.

Wes, I think POW reference values are done for axial viewing verification. The global compensation of FF lenses is another thing entirely, and cannot be vetted with conventional lensometry, unless you have the specific POW values for the specific point on the off-axis grid. And even then I'm not so sure, since the aperture size of a conventional lensmeter would be oversampling and, with operator interaction, averaging the zone being read.

B

[Wes]

Possibly, it depends on what you're looking to do. POW compensation is done to adjust for the aberrations induced by looking through the lens off-axis. The prescribed method for measuring these aberrations has been around for far longer than POW compensation.
I'll post a diagram and some more info when I get home.

Wes, I think POW reference values are done for axial viewing verification. The global compensation of FF lenses is another thing entirely, and cannot be vetted with conventional lensometry, unless you have the specific POW values for the specific point on the off-axis grid. And even then I'm not so sure, since the aperture size of a conventional lensmeter would be oversampling and, with operator interaction, averaging the zone being read.

B

I suppose that my part in bold wasn't very clear. Your part in bold is. Thanks. If your lens has positive (or negative) faceform, and pantoscopic tilt (more if a progressive) you are not looking through it axially, hence POW compensation. I suppose it didn't make much sense because of the various discussions going on.

[Barry Santini]

When the eye gazes straight ahead, this is *axial* viewing, but not to be confused with referring as the same as "off-axis", which is a more elementary reference for lenses using spherical curves, aka best form, away (aka "off") the optical center/axis of the lens. I beleive it is no longer a technically correct way to reference how FF, optimized lenses give up their gold.

B

[Wes]

Maybe we lack the terminology to discuss spherical best form lenses in the same paragraph as aspheric/freeform/POW lenses while retaining the "apples to apples" concept?
When I say axial viewing, I mean the following: a line of sight through a lens that passes undeviated through the optical center. If you drew a plane tangent to the optical center on the front of the lens, the line perpendicular to that plane represents the axial line. Rarely is that line concurrent with a straight ahead gaze, as almost all lenses have some amount of tilt and wrap. To have true axial viewing on any ophthalmic lens, one would nearly invariably be looking through one lens only, in a direction out and down. Is this, along with the changes in percieved power that result from both vertex compensation and this "non-axial" (my term) viewing not the "problem" that POW is meant to correct?
Also, I do understand that aspheric lenses are used to correct for the true off-axis aberrations of radial (oblique) astigmatism and power error, while providing a thinner, more low profile lens. New paragraph; see, I'm getting the hang of this.

[Barry Santini]

Good points, Wes. How about primary gaze angle as as a substitute term?

Remember that SV FF lenses, such as Auto II, can use yoked prism to reduce thickness, yet the eye gazes through the lens designer's intended reference point to correct off axis viewing better than BF, yet without any POW power adjustments, and away from the reference point of the traditional optical center/axis. So just where are the primarily and off-axis viewing points for well corrected lenses like these?

B