To the best of my knowledge, no one is currently offering a SV true aspheric CR-39 lens - digitally surfaced or otherwise. Can any here confirm or deny? Shy of that, is the next highest Abbe available going to be a 1.54 indice, then of course moving on to the likes of Trivex and so on down the regular list?
TIA
Of course you can get an aspheric CR lens. Auto II, Zeiss and Hoya all offer it in digital. In fact most of these aren't just aspheric, but atoric if there's cyl.
Define "aspheric".
By one definition, a +16 diopter front on a +4 diopter carrier with a 40 mm bowl is aspheric.
Do you want an aspheric front surface, or an aspheric/atoric back surface?
With only a couple of exceptions that I can think of, there are no Free Form designs that use a lens with an aspheric front surface.
Most Free Form Single Vision designs (in all materials) have an aspheric/atoric back surface, which is why you get a larger "clear" area of vision. As an example, my Autograph II SV AW lenses in Poly Drivewear (-6.75 -100 OD, -7.25 -0.75 OS) give me completely clear vision, nasal to temporal (53 eye frame).
At this point, as far as I can find, though I'm still checking a couple of vendors, the only aspheric CR-39 lenses still available are high base curves, 12.00 base and up.
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
http://www.bcdlens.com/products/asphericHRSV.aspx
The Hoya iD SV is bi-aspheric/atoric.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Front surface is what we're looking at. Sorry I didn't denote that in my earlier post.Originally Posted by WFruit
Mind me asking why? A front aspheric, ground on concave side with traditional generators will not give as accurate asphericity, for a given power, (nor any atoricity) as a FF design will. But if that's what you want, Robert pointed out Bristol offers it.
I think Bristol may be the only ones left, at least in the lower base curves. Signet Armorlite used to have them, but they're discontinued.
And, I have to ask this, why not poly? And I totally mean that in good humor
There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.
Especially when the base curves are not closely spaced (about 2 D with Bristol). The Bristol lenses might be flatter and thinner, but they might also have more oblique astigmatism than a flattened spherical lens at certain powers!
I agree that we're better off optically using a backside aspheric/atoric design for the vast majority of RXs, the exception being moderate to high plus where there are additional benefits when the majority of the asphericity is on the much steeper front curve compared to only the almost flat ocular curve, usually allowing a slightly flatter base curve with reduced weight and thickness, without forcing the use of a convex ocular surface.
The only question that remains is how long the lens manufacturers will keep these lenses in production considering inventory costs and the lack of demand due to the small pool of beneficiaries, and the lack of trained opticians who are well versed in the application of these more complex surface designs.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Rotational error will increase relative to variance in curves, as will polishing error. The combined avg error on high BC will effectively nullify any gains in Free-form, or even be worse.
There are a few choices in SV Aspherics, Essilor, Rodenstock, Nikon all offer SV Aspherics in CR-39.
The next highest ABBE after CR-39 (at 58) is the 46 ABBE of HiVex in 1.56 which is completely unlike other 1.56, although the formulation is a secret is seems that its both chemically and heat catalyzed, which makes it unique, but its my guess right now.
And many companies offer Free-form SV in CR-39...
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