# Optical Forums > Ophthalmic Optics >  When NOT to use Aspheric Lens?

## billtbyhand

Hi Everyone.

I am curious as to when you should not use an aspheric lens (other than possible adaptation problems from a patient's old lens)?

Is there a disadvantage to using aspheric lenses mild Rx patients?  I know that there will not be much advantage to having a flatter front base curve, but I don't know that it would hurt either.  Also, having a somewhat larger "sweet spot" around the optical center would be a good thing.

Also, why do we not see more aspheric lenses in CR-39 bi-focals.  I know that many times a patient could get by just fine on a moderate Rx with apsheric lenses instead of high index and there wouldn't be very much difference in the finished lens.

Just trying to activate a few brain cells:bbg:

----------


## QDO1

depends on 1 thing only - what sort of aspheric curve they used. the definition of Aspheric is : NOT Spehrical, and that is a broad definition of many lens designs

----------


## billtbyhand

Yeah, I guess that's right.  Technically all they need to produce is a lens with a 1mm larger radius and it's apsheric - just not very much.  

Speaking of which, any idea what kind of difference that you would see in a good aspheric lens?

----------


## HarryChiling

I personaly don't use aspherics on patients with prisms, and patients that are very sensitive to base curve changes (we all have those).  A good aspheric lens takes into consideration thickness as well as abberations.  The eccentricity of the lens will vary depending on if you want to reduce tangential and/or saggital errors, and will be different if you were to just making the lens as thin as possible.  I would suggest tinkering with Darryls Optical Analysis application it will give you a good idea of what the aspherics accomplish.

----------


## Alvaro Cordova

I would have to second what Harry said.  I would only add that you have to surface a lens if you are going to have any prism.  Do not decenter the lens in order to get the prism.

----------


## David Wilson

> Do not decenter the lens in order to get the prism.


Yes, this is important. In addition, though, if there is ordered prism then you must decentre the optical centre 0.25mm towards the apex for every prism dioptre. So if 2 prism dioptres base out is ordered RE then you must decentre 0.5mm in from the monocular PD. This also applies to progressives (and applies also to height). In the case of height you must drop the OC 1mm for every 2 degrees of tilt and then adjust for prism.

If fitted correctly an aspheric lens should behave equally as well as a 'best form' lens designed using Tscherning's ellipses. Asphericity merely allows us to use flatter, thinner lens forms. These flatter forms in spherical design would induce significant oblique astigmatism.

I also agree with Harry that it is worth trying Darryl's wonderful program.

Regards
David

----------


## drk

I would like to suggest that it is unwise to use an aspheric when you are doing a wrapped sunRx (I think I'm right on that one), the reason being that all the tilt formula's assume spherical optics.  I think that you would induce further unwanted (radial?) astigmatic error by tilting an aspheric.  Not only that, but I like to incorporate a small amount of prism Rx in the wrap sunwear.

I'm not sure I completely understand the problem with prescribed prism and aspheric lenses.  Is it that you should NEVER do prism with aspherics, or just that you shouldn't decenter aspherics to achieve prismatic effect?

----------


## Robert Martellaro

> I'm not sure I completely understand the problem with prescribed prism and aspheric lenses. Is it that you should NEVER do prism with aspherics, or just that you shouldn't decenter aspherics to achieve prismatic effect?


The latter, although I suspect you could decenter 2mm to get 1^ on a five diopter lens without screwing up the off-axis optics to any noticeable degree. Maybe. The only reason I can think of decentering instead of surfacing would be on a minus lens where using a finished blank would save time and money, and would allow the use of thin 1mm centers on lenses that would need 1.5mm or more if surfaced. Nevertheless, that sounds like a slippery slope leading to poor optics, and I have instructed the labs that I work with to always surface when there is prescribed prism with aspheric lens designs, and on plus powers for improved cosmetics and reduced weight.

Regards

----------


## billtbyhand

Thanks Everyone!
That does help me to understand this better.
Playing with Daryl's program also helps a bunch.  That is a really cool little piece of software.  Great job Daryl.

As someone who entered this field about 18 months ago after spending 22 years do computer installations and service, and having 5 days training with my local OD before turning the lab over to me to run!  :Eek:  
I've had a fun crash course in learning what this is all about.  

I have been working on a college degree in opticianry for the past year, but I have learned in many cases much more usefull stuff by reading through what goes on around hear.

As always, I am impressed with the level of professionalism and knowledge around here.

----------


## shanbaum

I'm not sure I completely understand the problem with prescribed prism and aspheric lenses. Is it that you should NEVER do prism with aspherics


No.


or just that you shouldn't decenter aspherics to achieve prismatic effect?


Yes.

----------


## drk

Thank you, wise Optiboarders.:bbg:

----------


## shahabsotoubadi

HI,
There is a question , can you please help me to understand the way of manufacturing aspherical lenses? if possible please inform me if any site with some pictures existed.thanks.

----------


## sharpstick777

\ 2nd Harry

added...

Some contact lens wearers prefer the steeper true form base curves you can only get with spherical lenses, and choosing the correct BC.  You cannot choose the base curve with aspherics.

Anesikonic lenses require choosing the base curve, so those are out.

Also, if the lens has to be ground (such as polarized), you are better going with SV Free-form because the usual 2, 4, 6 or 8 are too limiting.   standard base curves can suck and in aspherics they can be close to useless, as their is no way for them to full adjust the curves not knowing the actual power.  

Part of the aspheric design in a finished lens is compensating the micro base curves for the power, we can only do that either in finished SV or digitally surfaced FF lenses.




> I personaly don't use aspherics on patients with prisms, and patients that are very sensitive to base curve changes (we all have those).  A good aspheric lens takes into consideration thickness as well as abberations.  The eccentricity of the lens will vary depending on if you want to reduce tangential and/or saggital errors, and will be different if you were to just making the lens as thin as possible.  I would suggest tinkering with Darryls Optical Analysis application it will give you a good idea of what the aspherics accomplish.

----------


## sharpstick777

There are two types of Aspheric designs, and you really don't know what you are getting.  Button and full field (across the entire lens, or in a small central zone).

So I wouldn't recommend button types of aspherics for any type of prism in general, as the virtual lens could shift in higher prisms to the edge of the button.




> I'm not sure I completely understand the problem with prescribed prism and aspheric lenses. Is it that you should NEVER do prism with aspherics
> 
> 
> No.
> 
> 
> or just that you shouldn't decenter aspherics to achieve prismatic effect?
> 
> 
> Yes.

----------


## sharpstick777

> HI,
> There is a question , can you please help me to understand the way of manufacturing aspherical lenses? if possible please inform me if any site with some pictures existed.thanks.


They are 99% cast, which isn't too exciting.  Pouring liquid plastic into a mold doesn't warrant much YouTube hits.
There are few that are front surface digital, but blocking and polishing errors with current technology could put the lens out of power and nullify the effective curves quite quickly.

----------


## pezmaz

Just a couple points and correct me if I'm wrong......Flatter BC can lead to reflection problems for those in low power who are used to steeper curves. Also, if Px in high plus power readers there is possibility they won't reach expected acuity in aspheric due to reduced spec mag.

----------


## sharpstick777

if your using AR BC won't matter for reflection, but its why we never us a Zero base, we use a .5 BC for high minus.  AR should be on every high index lens anyway.  

Acuity will be better, esp in readers, because the magnification will be more equal across the width of the lens, resulting in better binocular vision.  




> Just a couple points and correct me if I'm wrong......Flatter BC can lead to reflection problems for those in low power who are used to steeper curves. Also, if Px in high plus power readers there is possibility they won't reach expected acuity in aspheric due to reduced spec mag.

----------


## pezmaz

thanks sharpstick........................i have to say we have had limited success with aspherics for reading. Patients genuinely feeling their new readers not as good as their old CR39's and seems lower spec-mag the most common observation. We are very careful in the dispense with measurements and certainly very few comebacks for distance vision. What views would you have on this?

----------


## sharpstick777

If an Aspheric is not performing well its usually an adaptation issue, people simply get used to their peripheral being weird and junky.  I remember when I switched the flat screen monitors, they looked bent to me for weeks.  My perception is what was bent.   The other possibility is binocular vision is reduced but they are simply noticing it more in clearer lens.

When troubleshooting, always, ALWAYS verify acuity.  I cant tell you the number of people who say they are not seeing well, but are seeing much better.  They are not nuts, it just feels different to them.

----------

