Aspheric design- cosmetic vs. optics

[Pete Hanlin]

All right. Yesterday I had a conversation with a very pleasant Essilor rep concerning asphericity. I was mentioning that I'm not a huge fan of aspheric minus powered lenses- unless the back side of the lens is aspheric. My reckoning is that the front side of a minus lens is already quite flat, but the ocular side of the lens could benefit greatly from "flattening."

Anyway, the rep said there is a difference between asphericity for cosmetic purposes and asphericity for optical purposes. I suppose I see where she's coming from, but when I pointed out that the Sola ViZio is a aspheric (or atoric, whatever the case) on both sides- while the Essilor lens is not- the rep said, "well, that's just for cosmetics- it doesn't help the patient's vision."

Okay, I'm not trying to pit one manufacturer against another, but it seems to me that SOLA has touted the ViZio as giving superior vision because of its bi-asphericity. In any case, I knew that I know someone who would know (whew, now THAT's a sentence), so I just smiled and nodded and made my way to my trusty laptop.

So, what's the story on bi-asphericity. Am I correct in assuming that a minus powered lens with an aspheric/atoric ocular surface is going to provide a greater benefit than one which is only aspheric on the front?

I understand that there is a difference in the type of asphericity used in different designs (aspherics with a "button" vs. full field aspherics and so on), but I'm specifically interested in ocular side asphericity here.

Pete

[Darryl Meister]

Hi Pete,

Before I get into this answer, I should clarify for the members of our studio audience that ViZio is only aspheric/atoric on the back surface -- not on the front (so it's not bi-aspheric). For single vision lenses like these, you can generally combine the optics of two surfaces into a single surface. Meaning, whatever you can accomplish with two aspheric surfaces you could also accomplish with a spherical surface and a single aspheric surface combining the surface curvatures of the two original aspheric surfaces. (So bi-asphericity isn't really necessary for these lenses, unless it afforded some other non-optical advantage -- such as a reduction in manufacturing costs or something.)

Anyway, you are right -- atoric lenses like ViZio provide better optics than aspheric lenses. It sounds like the lens rep might have been a bit unclear about the product -- or perhaps there was some miscommunication. An aspheric lens can only optically optimize for one power. When the lens contains cylinder power, which about 70% of all Rx's do, the optical performance has to be a compromise in an aspheric. Atorics, on the other hand, apply a unique amount of asphericity to each meridian. This allows atoric lenses to optimize completely lenses with cylinder power, which means that an atoric lens would provide a wider field of clear vision than a comparable aspheric lenses for lenses with cylinder power. For sphere powers, atoric and aspherics are pretty much the same (asphericity just becomes a special case of atoricity with an atoric lens series).

As for front versus back... Both front surface and back surface aspherics can be optimized to provide equivalent levels of optical performance. Some aspheric lenses are optimized for cosmetics, but those are much less common than the optically optimized aspherics you would buy in low to moderate powers. However, back surface aspheric lens designs are often thinnner in minus powers than front surface designs. Generally, when asphericity is applied to the stronger surface, the thickness is reduced a bit more than when it is applied to the weaker surface.

Best regards,
Darryl