An interesting question for thoughtful discussion

[rbaker]

Rereading some old research papers I came across this interesting paragraph:

"The concept on which current ophthalmic lens designs are based haveremained unchanged for many years.

It is generally understood that stock lenses of negative toric form possess advantages over those of positive toric form. Discussions and papers to this effect lead one to expect that such lenses will be available shortly.

It is also generally understood that geometric distortion is a defect that remains essentially uncorrected. Another paper, not yet published, points out that distortion can be more effectively controlled in the fitting procedure than by the lens designer. It will not be discussed further in this paper.

An Analysis of Ophthalmic Lens Design byJohn K. Davis, Henry G. Ferula & Arline W Rayner

The portion of the paper, quoted above, one of the first by American Optical concerns the work done by the authors on what was to become the Masterpiece™ series of lenses poses an interesting question for thoughtful discussion.

[Barry Santini]

The aberration of distortion is off axis in nature, and in its simplest definition is a variation of focal length with aperture. So the stop distance would matter.
B

[Dr. Bill Stacy]

The aberration of distortion is off axis in nature, and in its simplest definition is a varition if focal length with aperture. So the stop distance would matter.
B

When you say "stop distance" and "aperture" do you mean lens size? Most of us here are more focused on specs than camera lenses, so your jargon seems odd to me. Anyway, I have always felt that smaller eyesizes are almost always better than larger ones no matter what the Rx. I was happy to see the huge glasses go away and am sorry they are making a bit of a comeback. Anyway, pincushion and barrel distortions are rarely an issue in specs, as the article above pointed out so many years ago.

[lensgrinder]

When you say "stop distance" and "aperture" do you mean lens size? Most of us here are more focused on specs than camera lenses, so your jargon seems odd to me.

Not jargon. The term stop distance is used in lens design. It is the distance from the eye's center of rotation plus the vertex distance. This distance can be calculated and lens designed in "real-time" on free-form lenses. When the eye moves away from the center of a lens an error called "oblique astigmatism" is produced, which is a change in sphere and cylinder power. You reduce this error with aspheric back surfaces.

[Dr. Bill Stacy]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

I could be wrong, but I do get around and I've never heard those terms used in polite conversation. Anyway, you probably run in different circles than I do. I dont know any freeform lens designers personally. I do know about oblique astigmatism and that what was really meant in that post is there is "additional" or "unwanted" obligue astigmatism. In common terminology, oblique astigmatism refers to ordinary astigmatic error whose axis is nearer 45 or 135 than it is to 180 or 90, like my astigmatism, and has nothing to do with induced astigmatism from peripheral lens distortion.

[Barry Santini]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

I could be wrong, but I do get around and I've never heard those terms used in polite conversation. Anyway, you probably run in different circles than I do. I dont know any freeform lens designers personally. I do know about oblique astigmatism and that what was really meant in that post is there is "additional" or "unwanted" obligue astigmatism. In common terminology, oblique astigmatism refers to ordinary astigmatic error whose axis is nearer 45 or 135 than it is to 180 or 90, like my astigmatism, and has nothing to do with induced astigmatism from peripheral lens distortion.

Mmmm...not really

[Paul Smith LDO]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

I could be wrong, but I do get around and I've never heard those terms used in polite conversation. Anyway, you probably run in different circles than I do. I dont know any freeform lens designers personally. I do know about oblique astigmatism and that what was really meant in that post is there is "additional" or "unwanted" obligue astigmatism. In common terminology, oblique astigmatism refers to ordinary astigmatic error whose axis is nearer 45 or 135 than it is to 180 or 90, like my astigmatism, and has nothing to do with induced astigmatism from peripheral lens distortion.

Is it possible that your definition of "jargon" used by many of us Opticians is simply limited to an abbreviated understanding of what some, yourself included, may consider what an Optician to be. The vernacular used by many of us in the industry is acquired thru interactions with our clients as a means to illustrate A and P concepts with more simple examples; the human eye and a camera lens. It does not preclude us from a more complex understanding of lens design and the vernacular that follows the "science" behind it. The function following the form of the design.

[Dr. Bill Stacy]

Someone around here had a tag line something like "Jargon is the last weapon of the charlatan" or something like that. I think it's true and am constantly correcting employees who say something like "Now I'm going to take your P.D." Jargon. I tell them to say "now I'm going to measure the distance between your eyes". Now then the patient's been educated and not hornswoggled by the employee. This goes for docs, opticians and assistants. Same thing goes for "BP". A lot of people know what BP is, but how hard is it to say "blood pressure" just in case they don't?

As for my abbreviated understanding what an optician is, I think I know pretty well what an optician is in the USA. How many of them could describe what a stop distance is without looking it up?

Nor should they, unless they also happen to be lens surfacing software designers.

[Paul Smith LDO]

Interesting, so vernacular terminology specific to a group should not be used amongst it's membership for fear of ignorance. If you don't understand something research the subject ask questions until you comprehend. That is how we learn and many of us continue to do so. Simply put, I believe that our definition of what an Optician can and should be differs. If you are going to be a bear, be a Grizzly.

[uncut]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

I could be wrong, but I do get around and I've never heard those terms used in polite conversation. Anyway, you probably run in different circles than I do. I dont know any freeform lens designers personally. I do know about oblique astigmatism and that what was really meant in that post is there is "additional" or "unwanted" obligue astigmatism. In common terminology, oblique astigmatism refers to ordinary astigmatic error whose axis is nearer 45 or 135 than it is to 180 or 90, like my astigmatism, and has nothing to do with induced astigmatism from peripheral lens distortion.

The terms aperture, and stop distance are not unfamiliar to those of us trained as "ophthalmic dispensers" way up in the frozen north land............although we have been dumbed down and call ourselves opticians(tm) as of lately. Pleased to meet you, sir!

[Johns]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

If you would have taken Ophthalmic Theory with Dr. Norman Ross (an optometrist) in the late 80's, you would not only have heard those terms, but would have known them. So that's at least one OD that I know of who was familiar with the term.


Do you refer to temples as "handles"?

[TheRobotious]

All it takes is two posts and the thread is completely derailed by petty argument? I believe Mr Baker was hoping for some thoughtful discussion.

That being said, I agree with Mr Santini in that the stop distance matters, and that lens eyesize is a contributing factor as well (as Dr Stacy mentioned). Aside from that, can anyone suggest any other parameters that would have a significant effect on distortion? Parameters that could be influenced post lens design?

Certainly from the lens designer's perspective, there is not a lot that can be done to address distortion, is there? Asphericity and digital surfacing can reduce the distortion but even still the reduction is nothing major.

[Dr. Bill Stacy]

Do any of you gentlemen care to tell me exactly how you use stop distance in your occupation? And how do you go about measuring it?

[lensgrinder]

I think it's jargon. I don't know any optometrists or opticians who call the pupil an "aperture" or even know what the heck "stop distance" means.

Dr. Keating a professor at Ferris State Optometry School mentions aperture and stop distance is his book "Geometric, Physical, and Visual Optics".

I do know about oblique astigmatism and that what was really meant in that post is there is "additional" or "unwanted" obligue astigmatism. In common terminology, oblique astigmatism refers to ordinary astigmatic error whose axis is nearer 45 or 135 than it is to 180 or 90, like my astigmatism, and has nothing to do with induced astigmatism from peripheral lens distortion.

Oblique astigmatism is an aberration associated when view obliquely. This can be found in different text books, like the one above, along with Dr. Jalie's "Principle of Ophthalmic Optics", this article http://www.opticampus.com/files/opti...orm_lenses.pdf, Dr. Brooks(Professor at Indiana State University Optometry School) "System for Ophthalmic Lens Dispensing", and in this text book used by many Opticianry Schools http://www.opticampus.com/files/intr...mic_optics.pdf, and many more.

How many of them could describe what a stop distance is without looking it up?

Nor should they, unless they also happen to be lens surfacing software designers.

I would venture to say that more know about it than you think.
They should know how aberrations affect patients and which lenses correct for or reduce those aberrations.

Aside from that, can anyone suggest any other parameters that would have a significant effect on distortion? Parameters that could be influenced post lens design?

With free form a lens can be designed in real-time. The Rx can be analyzed to determine the eyes center of rotation, then added to the vertex distance. With this information along with taking the other PoW measurements, you will create the least aberatted lens of all.

Certainly from the lens designer's perspective, there is not a lot that can be done to address distortion, is there? Asphericity and digital surfacing can reduce the distortion but even still the reduction is nothing major.

Yes there is, again they are designed based on the PoW information that you provide. Take a +2.00 with the wearer looking off axis 30 degrees(15 mm), this induces approximately an 1/8 D of astigmatic error. The free form design can change for each power and vertex distance.

Do any of you gentlemen care to tell me exactly how you use stop distance in your occupation? And how do you go about measuring it?

Choose a lens that accounts for varying stop distances. Ask your lab if the lenses you use takes the eyes center of rotation into account.
You cannot measure the eyes center of rotation(the is determined from the Rx by the algorithm that you lab uses), but you can measure vertex distance.

[Barry Santini]

I hope that everyone in this discussion will try to keep to the orthodoxy that the aberration of distortion (rectilinear distortion) will not be confused with "unclear" optics, as in distorted or aberrated from power, coma or astigmatism errors.

OK?

B

[fjpod]

You guys are too deep for me, so I'll stay out of it.

[Paul Smith LDO]

Isn't the construct of the compensated Free Form design to address or clean up image displacement thru centrad, if so, would not stop distance be a critical value in the designer's pre/calculated logarithm, along with the POW and frame geometry, for a better aspheric or atoric lens design.

[Barry Santini]

Isn't the construct of the compensated Free Form design to address or clean up image displacement thru centrad, if so, would not stop distance be a critical value in the designer's pre/calculated logarithm, along with the POW and frame geometry, for a better aspheric or atoric lens design.

Yes

[lensgrinder]

Isn't the construct of the compensated Free Form design to address or clean up image displacement thru centrad, if so, would not stop distance be a critical value in the designer's pre/calculated logarithm, along with the POW and frame geometry, for a better aspheric or atoric lens design.

Most lenses were designed with an assumed stop distance, based on an average eye diameter and an average vertex distance.
Some free form lenses are still designed based this average center of rotation and then they design the lens based on the given vertex distance.
Stop distance is very important and some free form lenses can accurately calculate this value.

[rbaker]

I hope that everyone in this discussion will try to keep to the orthodoxy that the aberration of distortion (rectilinear distortion) will not be confused with "unclear" optics, as in distorted or aberrated from power, coma or astigmatism errors.

OK?

B

Perhaps geometric optics has changed but I was taught in Physics 101 at Dartmouth College back in 1958 that the principal lens aberrations are:

Spherical aberration
Chromatic aberration
Oblique astigmatism
Distortion
Coma
Curvature of field

As I recall, some 57 years later, two of the abberations are expressed both on the optical axis and off the optical axis, those being spherical aberration and chromatic abberation while all the other abberations occur off the optical axis.

In ophthalmic optics we need only be concerned with spherical abberation as it is the only abberation that can be reduced by a single lens design, and not a doublet, of an aspheric design thereby forming a clear image at the same point in the image plane.

I would be very interested in learning about any new developments in the field of geometric optics for ophthalmic applications that you would care to share. Pleas go easy on me as I aint nearly as sharp as I was back in the fifties.

[Barry Santini]

For astronomical use, we have designed eyepieces with over a 100 degree apparent FOV, with minimal astigmatism and lateral color. When mated to an objective where the focal planes complement each other, then pin point star images are the result, even if the combined focal plane departs from flat (visually). However, this is accomplished by correcting angular distortion, and allowing geometric distortion to prevail, because simultaneous correction of both types is impossible using conventional optics. Good in the sky, where most objects are pin point stars of negligible field angle, and bad terrestrially, where ground optics used to spot the enemy would be considered unusable if the apparent location of the enemy from distortion or distorted parallax was present.

B

[Barry Santini]

Also:
http://www.lensrentals.com/blog/2010...ly-aberrations

Try this link for a primer on the aberrations Dick cited above.
Scroll down to distortion and note the discussion of the effects of stop position, which effectively limits the size of off axis ray bundle.

When you think about it, distortion is a variation of magnification across the aperture. And Magnification is related to focal length. So distortion is a example of a geometric object being imaged by a system suffering from Spherical aberration.

B

[Paul Smith LDO]

With regards to question 13. I can see the importance of the lens designer wanting control with regards to the stop distance either lengthening or shortening it depending upon the required lens curvature, Rx, and vertex to produce an effective lens design. I can't image that they would want to use numbers obtained thru some else's statistical information, as this is all propitiatory.

[Robert Martellaro]

In ophthalmic optics we need only be concerned with spherical abberation as it is the only abberation that can be reduced by a single lens design, and not a doublet, of an aspheric design thereby forming a clear image at the same point in the image plane.

My understanding is that spherical aberration is generally not an issue with ophthalmic lenses due to the small pupil size.

WRT distortion, as Barry implied, pincushion and barrel can only be minimized by decreasing the lens diameter and vertex distance.