# Optical Forums > Ophthalmic Optics >  Single Vision Design Continuum: Spheric, Aspheric, Free Form

## RKJ

*Do we have any simple pictures explaining how single vision surface curves change as we move along a design continuum from spherical to aspheric to free form, for: plus lenses, minus lenses, and spherocylinders?* 

*I am trying to understand, in very simple terms, single vision lens curves.*



*This is what I understand so far:*
*Basic optics* - The patient experiences oblique astigmatism when looking through the periphery of a lens with spherical front and back curves.
*Plus Aspheric* - Plus aspheric single vision lenses are generally front surface aspheric  _steeper in the center and flatter in the periphery_  designed to minimize oblique astigmatism.
*Minus Aspheric* -Minus aspheric lenses are generally ___( back?)___ surface aspheric  _flatter in the center and steeper in the periphery_. (I think this is true.) *This is what I dont understand:* 
*Plus Lens:* _If the majority of single vision free form designs have a spherical front surface and free form back surface_, what is the general shape of the free form back curve?  Where is it steeper and/or flatter than a sphere?
*Minus Lens:* For a minus lens, how different is the computer generated free form curve, or set of curves, than the curve(s) we have with lab choice aspheric single vision lenses? In general, how is the shape of the free form curve different than the aspheric curve? Or, how is the free form back lens surface shape different than a common aspheric back lens surface shape?Recently, a lab representative shared that only a few PHD educated folks really understand the lens curves related to optics. It seems to me that understanding the basic shapes of plus and minus lens surfaces is not Rocket Science so much as Proprietary. 

I am embarrased to admit I don't understand how single vision lens shape is changing over time to reduce optical aberation. I hope someone at this forum can help.

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## HarryChiling

Here's an animated gif I made.

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## Barry Santini

Nice GIF, Harry.

I suggest you look at Darryl's opticampus.com entry on ophthalmic lens design.  It's really all there!

Barry

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## HarryChiling

> Nice GIF, Harry.
> 
> I suggest you look at Darryl's opticampus.com entry on ophthalmic lens design. It's really all there!
> 
> Barry


http://www.opticampus.com/cecourse.php?url=lens_design/

Yeah he's got a knack for making a difficult subject childs play.

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## Darryl Meister

Please note that I've moved the posts related to opticiany and optometry requirements to a separate thread.

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## RKJ

Hi Harry and Barry - Thank you both for your help! I read Darryl Meisters opticampus lesson  exactly what I needed! I found some answers, but not all, and have new questions too. 

I was slow to respond because I have been out of the country, away from my computer.


What follows is first, a lengthy explanation of my agenda. This is followed by what I learned from your previous recommendations, then questions I still have, and a few brief statements - predictions about how new technologies bring opportunities and some of the same old challenges. I wonder what you think and look forward to additional advice regarding where I should read/inquire next to understand optics.

*I now understand the design continuum from spheric to aspheric, but not from aspheric to free form, specifically for single vision: plus lenses, minus lenses, and spherocylinders.* 

*Ambition:*
*My real ambition is to create a set of pictures and words illustrating single vision lens technology advancements; a simple sales tool that accomplishes a set of goals:*

*1.* Education - Patient education is key to high end sales.
a. Doctors need a few simple sentences to use when recommending free form. This tool will help the doctor.

b. Opticians educate patients. This same tool will help opticians and the doctor provide consistent product messages to patients - throughout the office, year to year, even with multiple providers, even during periods of staff turnover._In my opinion, saying High Definition is inadequate because:_ 
1. It is not entirely true. 
2. It limits possibilities for continuing patient education through encounters over the years. 
_Guerilla marketing is consistent marketing._ 

The educational sales tool document should be true, easy for a lay person to understand, a couple of pictures and a couple of concise statements and several benefit statement bullet points. We train our patients to expect changes in lens technology. Lenses are technology items. Technology advances over time. Patients learn from multiple presentations, of identical pictures and words, over the years. If the sales tool shows technology advances as a continuum, the same pictures and words can be used in the future. We simply add the latest and greatest to what the patient has already seen. Re-education and new education will be easier. *2.* As a result, within an office: 
· Doctors are on message.
· Opticians are on message.
· Patients learn to buy the best. 
· The optical turns faster. For existing patients, it takes seconds to flash the past, and new recommendations are based upon advances in technology since the patients last visit.
· Educated patients leave the office with the ability to explain why their lenses are better. Educated patients influence friends and family.*From Darryls course, I found this content - simple statements, true and easy to illustrate**:* 
· *Plus lenses.* If asphericity is applied to the _back_ surface, the surface will become _steeper_ away from the center.

· *Minus lenses.* If asphericity is applied to the _back_ surface, the surface will become _flatter_ away from the center.
*I still have questions**:*

1. What is the lens shape difference between lab choice aspheric lenses versus free form - for plus, minus, and spherocylinders
· *How* *different* is the computer generated free form curve, or set of curves, than the curve(s) we have with lab choice aspheric single vision lenses? Or, how is the free form back lens surface shape different than a common aspheric back lens surface shape?

· *Specifically* *what is the new shape as aspheric goes to free form for minus lenses? For plus lenses? Again, just considering single vision.*I want simple true statements and/or illustrations _any_ optometrist, optician, _and patient_ can understand with ease. 

*2.* This next set of questions is easier to ask with pictures, because my vocabulary regarding optical errors is shaky. *I am struggling to understand reasonable expectations for vision improvement.* I think these are power error questions, about optical limitations that are not solved with new technology. *Do I understand this correctly? If not, where is my optical understanding flawed?*

*SEE ATTACHED JPG*

A. For a spherocylinder lens, only the optical center has the accurate prescription. 
§ Even a free form lens does not solve this problem. 

§ As a result, the higher the cylinder, the smaller the sweet spot of good vision.B. The prescription is most accurate along the major meridians. 
§ Therefore the sweet spot of best vision is probably as drawn here, some shape of a 4 point star. 

§ A patient with WTR or ATR astigmatism will have a wider horizontal sweet spot of clear vision than the patient with oblique astigmatism.

§ As a result, the patient who has oblique astigmatism is more likely to be a head pointer than someone with WTR or ATR astigmatism. The higher the cylinder, the more this is true.

§ Free form lenses do not solve this problem.3. Tools exist to measure a patients eye movement behavior. If we all had those tools, and knew how to use them, most likely we would discover that patient's head and eye movements have been trained over the years due to the optics of their prescription combined with visual requirements of life activities. 

*Our FUTURE**: I am trying to understand the present and plan for the future. I dont need to explain this to patients. Feel free to respond to any of the following statements? Please recommend resources for reading.*

If I understand the future of our developing technology, with software algorithms, robotic technology, and the ability to assess a patients head and eye movements during work, leisure, and play, then the expectation is that new lens designs will be more and more customized to the patient.

If I understand optics, even with all of the advances currently in process, patients will still need multiple prescriptions for best vision during work, leisure, and play. The two top indicators for best lens topography will continue to be visual demands during a given activity and the prescription. 

Furthermore, because head movement and eye movements are 1. Hard to measure, and 2. Trained by optical limitations over a lifetime, Even given the advances possible with our new technologies:
· We will probably continue to use the two top indicators for lens design: activity visual demands and prescription. 

· Providers might get better at POW measures. Much of this is smoke and mirrors; the default measures are pretty good for low to moderate prescriptions.

· Probably, best lens topography, per patient, will be similar to designs we already have. 

· Given that best lens topography is likely to remain proprietary, doctors and patients will continue to select based upon the pitch by the local rep and patient experience. We probably wont have lens topography maps or other tools to help us compare the brands.

· We have new problems in that our office tools  lensometer, calipers, lens clock  are no longer adequate even to verify that free form prescriptions are manufactured accurately.

· Perhaps we will get better at educating patients regarding choices.
This is a great time to be in the optical business. Challenges and opportunities are abundant. 


Now I have come full circle and find myself back at the task of creating a one page set of simple pictures, statements, and benefit bullets  simple, easy to add content over time, ready to take my patients and my business into the future.

Thank you in advance for advice, comments, and resources you might share!

Renee

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## RKJ

Dear Darryl,

Thank you for all you do.  I applaud your opticampus coursework!

I replied to Harry and Barry, but also want your advice/direction/instruction.  I need a better working understanding of opthalmic optics and recognize that you are an expert.  

Cheers!
Renee

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## Darryl Meister

> Thank you for all you do. I applaud your opticampus coursework! I replied to Harry and Barry, but also want your advice/direction/instruction. I need a better working understanding of opthalmic optics and recognize that you are an expert.


Renee, I will attempt to answer a few of your questions briefly:



> What is the lens shape difference between lab choice aspheric lenses versus free form - for plus, minus, and spherocylinders


By "lab choice" aspheric lenses, I assume that you mean factory-molded aspheric and/or atoric lenses.

Factory-molded semi-finished aspheric will generally utilize a rotationally symmetrical aspheric convex surface for the front of the blank that becomes either steeper away from the center for base curves used with minus prescriptions and flatter away from the center for base curves used with plus prescriptions.

Factory-molded finished or stock aspheric/atoric lenses may utilize either convex aspheric surfaces for the front of the blank or concave aspheric/atoric surfaces for the back of the blank or a combination of the two, depending upon the particular lens design.

For the front surface, stock lenses will adhere to the rules mentioned above for semi-finished aspheric lenses. For the back surface, the curvature flips: The surface becomes flatter away from the center in minus prescriptions and steeper away from the center in plus prescriptions.

If there is cylinder power and asphericity is applied to the back surface, the back surface will be _atoric_ instead of _aspheric_. Whereas an aspheric surface has rotational symmetry about the center of the design, an atoric surface has two mutually-perpendicular planes of symmetry.

The particular "shape" of free-form lenses generated by the laboratory will depend upon several factors, including the level of optical customization offered by the manufacturer. In many cases, an aspheric or atoric surface is generated onto the back surface of a spherical semi-finished lens blank or "puck."

If more complex optical customization is utilized, including customization for the position of wear, a more complex back surface is produced. Such a surface is more "general" than aspheric and atoric surfaces, lacking any form of symmetry, and also more difficult to fabricate.

Zeiss Individual Single Vision, for instance, applies point-by-point optical customization for both the prescription powers and the position of wear, which results in varying atoricity at each point over the entire surface.

Regarding the potential visual benefits of free-form single vision lenses, I would strongly encourage you to download and read the Zeiss Individual Single Vision white paper from the www.vision.zeiss.com website. It gets into more detail regarding the potential improvements in vision quality afforded by fully customized lens designs.

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## RKJ

Dear Darryl J. Meister, OBOM -

This is great!  I am getting the idea now.  Between the time I started this thread, and the time you replied, I rephrased my question and it took a little time to post, and there are two great replies.  Please take a look at those and let me know what you think.

Do you have any ideas where we can get abberation maps exactly like those shown at the Seiko site, but for a variety of lens spherocylinderical powers, plus and minus.

Sorry, I don't know how to point you to another thread.  I'm pretty new to this.  Please write back if you can't find it.

Cheers!
RKJ

PS, I did go towww.vision.Zeiss.com and had difficulty finding the Zeiss Individual Single Vision white paper .  Can you please share the path to the document?

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## rinselberg

RKJ--I think this is the document that you are looking for.

http://www.personalizedlens.com/imag...tePaper_B2.pdf

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