# Optical Forums > Ophthalmic Optics >  Base curves for spherical lenses

## VVizard

Hello.

I am wondering is there any common practice for base curve selection for spherical SV lenses from practical\commercial point of view. 

What is your way to do this? How it depends on index of refraction?

Thanks.

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## the dog

:idea: MY PHILOSOPHY IS THE FLATTER THE BETTER.  THE INDEX OF REFRACTION MAKES THIS POSSIBLE DUE TO THE FACT THAT THE 1.589 INDEX COMPENSATES LESS THAN 1.498.  FOR INSTANCE 6 BC 1.498 WITH A POWER OF +3.00 WOULD BE AROUND 2.87/2.75.  A POLY WITH THE SAME BC AND POWER WOULD NEED A 3.87 CURVE.  i HOPE THIS IS THE ANSWER YOU'RE LOOKING FOR.  GOOD LUCK !!!

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## Darryl Meister

Article on Lens Design

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## VVizard

Thanks for the article, Darryl. I do know the best curve theory, what I was refering to is the actual practice. Because, does anybody really make +2.25D prescription in 8D base curve? Does anyone uses 0.5D base curve?

I want to make some kind of survey on this. How much cosmetics pushes optics nowdays?

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## Darryl Meister

> I want to make some kind of survey on this. How much cosmetics pushes optics nowdays?


I'm sure cosmetics pushes optics quite a bit for those who don't understand the optics.

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## Darryl Meister

> I want to make some kind of survey on this. How much cosmetics pushes optics nowdays?


I'm sure cosmetics pushes optics quite a bit for those who don't understand the optics. However, prudent manufacturers try to retain the optics, and use tools like asphericity and high-index materials to improve the cosmetics.

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## HarryChiling

If you were to use Darryls software in the download section you can see how much off axis errors are created when the base curve differs from best curve.

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## rbaker

The scientific work of Dr. Tillyer at AO and the folks at B&L and Zeiss in the 1920s and 1930s is as valid today as it was then. More importantly, it has never been improved upon. Why anyone would want to deviate from this corrected curve lens form is beyond me unless visual acuity is of secondary concern.

Read the works of these lens pioneers and all will become clear (pun not intended.)

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## VVizard

Darryl, rbaker or anyone else, do you have good available reference for derivation of Tscherning ellipse?

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## lensgrinder

Here are two great articles that discuss Tscherning's Ellipse. There are also other books that will reference it. I would also highly recommend using Darryl's Optics Lite to gain a better understanding.

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## VVizard

I am looking for the formula of Tscherning ellipse.

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## OPTIDONN

> I am looking for the formula of Tscherning ellipse.


That will be tough. I believe that you would have to ray trace every base curve combination then once you find which base curve results in the least amount of off axis aberrations is the one you would want to go with. Of course this is a very watered down explanation. There is no formula for Tscherning's Ellipse. It's based on two base curve theories one by Ostwalt and the other by Wollaston. Tscherning created the ellipse to illustrate both theories.

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## OPTIDONN

> :idea: MY PHILOSOPHY IS THE FLATTER THE BETTER. THE INDEX OF REFRACTION MAKES THIS POSSIBLE DUE TO THE FACT THAT THE 1.589 INDEX COMPENSATES LESS THAN 1.498. FOR INSTANCE 6 BC 1.498 WITH A POWER OF +3.00 WOULD BE AROUND 2.87/2.75. A POLY WITH THE SAME BC AND POWER WOULD NEED A 3.87 CURVE. i HOPE THIS IS THE ANSWER YOU'RE LOOKING FOR. GOOD LUCK !!!


 :Eek:   huh! If your not concerned about optics I would agree that flatter is better. Thankfully manufacturers have designed aspheric and atoric lenses that allow us to maintain some optical quality with a flatter design. But I would not agree that flatter is better.

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## OPTIDONN

> Thanks for the article, Darryl. I do know the best curve theory, what I was refering to is the actual practice. Because, does anybody really make +2.25D prescription in 8D base curve? Does anyone uses 0.5D base curve?
> 
> I want to make some kind of survey on this. How much cosmetics pushes optics nowdays?


I use these curves all the time.

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## shanbaum

> I am looking for the formula of Tscherning ellipse.


 
http://www.umsl.edu/~swlong/download...ens_Design.pdf

See p. 5

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## jofelk

> I'm sure cosmetics pushes optics quite a bit for those who don't understand the optics. However, prudent manufacturers try to retain the optics, and use tools like asphericity and high-index materials to improve the cosmetics.


We have found that not only cosmetics but more importantly FRAME DESIGN, wraps and smaller, long rectangular shapes have caused us to change the base curve. It is necessary to flatten or raise base curves to get proper fit. The CORRECT base curve does not work in alot of todays designer frames.

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## Fezz

> The CORRECT base curve does not work in alot of todays designer frames.


 
Jofelk makes a very good point. I am convinced that the wingnuts who design alot of todays frames have absolutely zero clue about optics and what it takes to mount a lens SECURELY in a frame. I try to weed out poor designs during selections for our stock. I feel really bad for these poor lab folk that do not have a choice. I guess the advances in edger designs has helped with the tracking ability and bevel placement options.

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## OPTIDONN

It's a good thing I have never attended a vision expo, cause if I did and I saw a frame designer there I feel I would have to be restrained!! Some of those frames look good but can't hold a lens no matter how tried.

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## jofelk

> ... I guess the advances in edger designs has helped with the tracking ability and bevel placement options.


  I can not fathom doing alot of todays frames with old edgers.

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## rbaker

So, what you are saying is that the resulting visual acuity that a customer receives is more a function of the frame designer than the optician. Do you allow the customer to select any frame, no matter how outrageous, and then do whatever you have to do to the lenses in order to get them to fit?  Apparently a lot of you do believe that function follows form. 

x

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## Fezz

I was lucky. I got my start in a wholesale lab. I was an eager to prove myself teenager who got all the great edge jobs, pattern making (by hand), and facet garbage that the nice folks(we call them "mentors" now) felt I needed to learn to do. I can still remeber some of the frame styles that would never hold a lens, proper base curve or not!

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## OPTIDONN

> So, what you are saying is that the resulting visual acuity that a customer receives is more a function of the frame designer than the optician. Do you allow the customer to select any frame, no matter how outrageous, and then do whatever you have to do to the lenses in order to get them to fit? Apparently a lot of you do believe that function follows form. 
> 
> x


I agree! It's just hard when the store manager or owner is mor concerned about giving the patient what they want regardless of how it will work.

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## HarryChiling

Unfortunately to the sales staff in the front of the house making the sale is much more important than optics.  So the patient will always flock to what does not work in their Rx.  A good optician will guide the patient into a better fit frame or at least a better compromise.  I like to take a look at the patients facial structure and the Rx before I let them peruse the frames so I can give them a recommendation as to what is going to look good with the Rx.

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## shanbaum

Tough problem, isn't it?  On the one hand, the competent optician wants to tell the customer that he can't have _that_ frame because of his Rx; but chances are, there's another guy up the street - who may be less competent, or merely less principled - who will do the job for him.

I'll call it "The Rumsfeld Paradox".

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## Darryl Meister

The good news is that lens designers are aware of these problems, and have made various design options available in order to address them, including aspheric lens designs for flatter frames and special atoric lens designs for steep wrap frames.

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## HarryChiling

> The good news is that lens designers are aware of these problems, and have made various design options available in order to address them, including aspheric lens designs for flatter frames and special atoric lens designs for steep wrap frames.


Good point, aspherics do help alot with the smaller B frames and we do charge a premium for aspherics so the patient is happy and so are we.

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