Double Aspheric lenses

[kirkafur]

I'm an optical retailer, and our computers only allow us to enter a double aspheric lens if the cyl is equal to or greater than 1.00. If it is under, it will only let us enter a regular aspheric lens.
The only lens we have in double aspheric is Trivex.

My question is whether a double aspheric lens would adversely affect anything if the script has a low or nonexistent cyl.

[WFruit]

It will not adversely affect anything. On the other hand, the benefits will be negligible as well. I'm guessing you're using Augen's lenses yes?

[DragonLensmanWV]

Not at all - it will be better in almost every case, just maybe not so noticeable in lower cylinder powers.

[gunner05]

Work for ECCA?

[WFruit]

Work for ECCA?

That was my guess as well. They were just starting these when I was with them. I think the contest winner for "name the lenses" was something like "Eagle Vision 4.0" or some such nonsense.

[Mylok]

I have worked with those Augen lenses before. As I understand it they have approximately 1/3D steeper curve 90 degrees from the base meridian. Thus the "double aspheric" designation.

Correct me if im wrong, but as I understood the recommendation of nothing below a 0.75 cylinder was due to this front side curvature. You would have to cut the back to a curvature that would negate the cylinder present on the front to create a sphere. When I first started working with those lenses I did try to make a sphere work on a double aspheric and did get close to the RX but not nearly to the clarity I could when just using the standard aspheric version.

So unless someone knows something I dont I would suggest sticking to the recommendations with the double aspheric. One thing though cylinders over the 0.75-1.0D that they want on a double do work fine on a single. You just sacrifice the optical benefit these high cylinder patients would have received from the double. From my years of working with them i would have to say that the percentage of patients that would actually notice the difference is almost non existant.

[Jacqui]

These are really just the OLD plus cylinder type lenses that should have been discontinued and forgotten in the 50's or 60's

[Darryl Meister]

Generally, a "double aspheric" lens is just a lens with some combination of asphericity on the front surface and atoricity on the back surface. The back surface will only be atoric in prescriptions with cylinder power. Overall, the optical and mechanical differences between a "double aspheric" lens design and a conventional single aspheric surface will depend upon the design choices and level of sophistication of either lens design.

Obviously, a lens with an atoric surface will provide better control of peripheral optical aberrations in lenses with cylinder power. For lenses with sphere power only, the optical differences between a "double aspheric" lens design and a single aspheric surface are negligible.

You can improve cosmetics if you judiciously control the distribution of asphericity between the front and back surfaces, which would be more likely with a "double aspheric" lens design. Plus lenses will be thinner with aspheric front surfaces, for instance, whereas minus lenses will be thinner with aspheric back surfaces. Not every "double aspheric" lens design does this though.

In either case, "double aspheric" certainly doesn't imply that the lens is "twice as good." Hopefully, the lens manufacturer can provide you with specific technical details regarding the design of the lens.

Best regards,
Darryl

[kirkafur]

Thank you, Darryl; your answer actually made sense to me.

[trifoil]

is a surface that combines only simple sphere and simple cylinder, considered 'aspheric'?

[Darryl Meister]

is a surface that combines only simple sphere and simple cylinder, considered 'aspheric'

It sounds like you're describing a toric surface.

Best regards,
Darryl

[trifoil]

Higher-order-curves, are curves beyond sphere and cylinder, such as ellipse, parabola, and hyperbola. Is there any application for a surface that?, uses a certain set of higher-order-curves in one meridian, and a different set of higher-order-curves at 90 degrees

[Darryl Meister]

Is there any application for a surface that?, uses a certain set of higher-order-curves in one meridian, and a different set of higher-order-curves at 90 degrees

What you're describing is an atoric surface.

Best regards,
Darryl

[sharpstick777]

Darryl, there a few lenses that are really double Aspheric on the front and back, not just atoric, Seiko and Hoya both have one. But you are correct most of the time. It makes sense though as Asians have the highest rates of extreme myopia.

A double aspheric (for spherical compensation only) only makes sense in very high plus or minus powers, when the actual base curve is very far from the true base. For example, if I ordered both a 1.74 -9.00 1.74 and a -14.00, they will probably be processed on the same .5 BC aspheric blank (we usually don't get into bi-convave lenses until -16.00 or more). However, in reality, the higher sphere patient will need different degree of asphericity for optimal vision. So optimally if we used a cast aspheric front adding a custom digital backside aspheric design could improve vision. In theory at least.

The farther we deviate from Vogel's Rule the need for a double aspheric lens increases. I don't see much advantage however in any power under -9.00 or +5.00, but it could depend on what base curve is needed vs. what the base curve should be, the type of aspheric design (full field or button) could also have an impact.

[chip anderson]

Don't know why it never ocurred to me until now, but any toric lens or surface meets the defintion of : Aspheric. Any spherical lens meets the definition of Atoric.
Maybe we need better more explisit words.

Chip

[Darryl Meister]

Darryl, there a few lenses that are really double Aspheric on the front and back, not just atoric, Seiko and Hoya both have one. But you are correct most of the time.

All so-called "double Aspheric" lenses are actually atoric on the back surface. The vast majority of prescription combinations will contain cylinder power, which will necessitate a toric back surface. Any asphericity applied to the back surface will therefore result in atoricity. In the handful of SKUs that have only sphere power, a rotationally symmetrical surface will be applied to the back of the lens, resulting in an "aspheric" back surface, which is practically speaking just a "special case" of atoricity.

However, in reality, the higher sphere patient will need different degree of asphericity for optimal vision... For example, if I ordered both a 1.74 -9.00 1.74 and a -14.00, they will probably be processed on the same .5 BC aspheric blank...So optimally if we used a cast aspheric front adding a custom digital backside aspheric design could improve vision.

Yes, each prescription power requires a unique lens design to minimize optical aberrations fully. But, No, a second aspheric or atoric surface is not required to accomplish this. Very similar optical performance can be achieved with a single suitably chosen lens design on either the front surface or the back surface of the lens.

Your example is just an illustration of the limitations of semi-finished lens blanks, which are only available in a handful of front curves. Finished lenses are not necessarily limited this way. Further, if you are "digitally surfacing" the back surface with an aspheric or atoric design ayway, there is no optical need for an aspheric front surface. The optical requirements of the lens design can also be accomplished with a single surface on the back of the lens.

That said, manufacturers of fnished stock lenses still use a finite number of front and back molds, so different prescription combinations may share certain front or back molds. In this case, the distribution of asphericity between the front and back surfaces can be optimized to balance optical performance with the smallest number of molds. And, again, there are certain mechanical and cosmetic benefits to the use of asphericity on the front versus the back surface of the lens, or vice versa, in certain prescriptions.

Best regards,
Darryl