Distance Zone Designs of PALs.

[Chul]

Hi everyone.

As you know well, there are three types in PALs distance zone design. they are spherical, spherical to aspheric, and aspheric.

I can understand first one and last one. however, I can't understand second one.

please let me know.

[OPTIDONN]

To be litteral all progressive lenses are aspheric. From what I understand of what you are asking, any aspheric lens has a small central portion that is 'spherical' as you move further away from this central area the surface curvature changes. So as far as I know the "aspheric, spherical to aspheric" lenses are one in the same.

[Darryl Meister]

Unfortunately, the term "aspheric" is used very loosely in progressive lens design.

Originally, the term referred to the early second generation progressive lens designs that distributed unwanted surface astigmatism well into the distance region -- changing the distance from a relatively spherical surface into a significantly more non-spherical (hence, "aspheric") surface. This design philosophy would be similar to the so-called "softer" lens designs of today.

However, the term "aspheric" in the context of modern progressive lenses generally refers to the use of local asphericity in the central distance and/or near viewing zones to improve optical performance on flatter Base curves or for the position of wear. This is similar in principle to the use of asphericity in single vision lenses. Unfortunately, some manufacturers have occasionally exploited the ambiguous meanings of the word "aspheric" in the past by referring to their traditional lens designs as aspheric (after all, a progressive lens is not a sphere) to imply that these designs have more optical optimization than they really do.

[Chul]

Thanks a lot OPTIDONN and Darryl Meister.

I also know a few manufacturers claim that their PAL has a aspheric distance. Dr. Borish said that all PALs are "rotationally aspheric". here, what means the rotationally aspheric?:hammer:

Darryl, does Gradal 3 has a aspheric distance? also multi design? or not?

I'm looking frward to hearing your kind answer.

[Darryl Meister]

Gradal 3 is both aspheric in the "soft" sense of the word and in the sense of being cosmetically flattened and locally optimized for the as-worn position. (All Zeiss progressive lenses are designed with this type of optical asphericity.)

Progressive lenses are not really "rotationally aspheric," at least in the typical mathematical sense of the word, though early progressive lenses were designed with cross-sections that were conic (aspheric) in shape. Nowadays, most are surfaces of greater complexity though.