base curve for progressives

[eyeboy]

I might be really thick here but why are progressives put on steeper curves than the similar prescription in single vision.

[Darryl Meister]

They're generally not, at least compared to traditional single vision lenses, though the steeper progressive near zone may make them appear as though they are.

[eyeboy]

I suppose what I meant was... . If say a +2.00DS with a 2.00 ADD varifocal was produced on a 6.00 base would this have less distortion than if it were worked on a 4.00 base?

[Darryl Meister]

Really, all +2.00 lens types would have less distortion when placed on a 6.00 Base, unless the lens was highly aspherized.

[eyeboy]

When you say aspherised what do you mean. I thought all varifocals were aspheric... but in this context I suppose they mean none spheric not ellipsoidal?

[Darryl Meister]

It's really somewhat of a misnomer to call progressive lens surfaces "aspheric." The word is usually reserved to describe surfaces that depart from a perfect sphere in a predictable -- and, often, rotationally symmetrical -- way. This isn't really the case with progressives. Similarly, when a substantial portion of the astigmatism produced by a progressive lens surface is moved into the distance region in order to soften the design, these lenses are often called "aspheric" progressives.

However, the word "aspheric" in the context of progressive lens design generally refers to asphericity applied locally in the distance zone (and, sometimes, near zone) to improve off-axis performance when the Base curve of the lens design has been significantly flattened from traditional "best form." This is similar in principle to the asphericity used in thinner, flatter single vision lenses.

[dbarenti]

I am trying to understand how some labs come up with the best-form base curve. It was my understanding that you would add the sphere to the cylinder to determine the best-form. For instance -3.00 -1.50 85 would give you a 4.5 base curve. Am I correct in my thinking.

Thanks,
David

[Darryl Meister]

I would suggest reading the section called "Base Curve Selection" in this article: Ophthalmic Lens Design. It covers the basics of Base curve selection, and also provides common prescription ranges.

[dbarenti]

Darryl,


I appreciate the link to Ophthalmic Lens Design. It had a lot of good information but not what I am looking for. I am a software engineer and I am looking for formulas that I can use in a program. From what I have found Vogel’s Rule is the best formula available.



For plus powers:

Base = sphere + 6.00

For minus powers:

Base = sphere / 2 + 6.00



That part is easy but when you have a cylinder will the following be correct?



For plus powers:

Base = (sphere + (cylinder / 2)) + 6.00


For minus powers:


Base = ((sphere + (cylinder / 2)) / 2) + 6.00

[Darryl Meister]

Lens designers don't use Vogel's formula to determine Base curves. However, if you're simply trying to approximate best form Base curves, it will come fairly close in most instances.