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Thread: Aspheric Spectacle Lens Useful Zone

  1. #1
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    Stick out tongue Aspheric Spectacle Lens Useful Zone

    Hi, I have been wondering if there is an effective minimum diameter at which an aspheric lens loses it's advantage. The aspheric surface is most spherical near the center and the curvature gradually flattens out further from the center. So, if the diameter of the lens were say the size of a quarter, you would not be getting as much into the more aspheric zone than if the lens were a couple of inches in diameter. In other words, it seems to me that as the diameter of the lens increases, the usefullness of the aspheric lens increases, and vice versa. So is there a minimum diameter at which the benefit of asphericity is nil? I would be interested in others thoughts on this.

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    Really hard to say, there are aspheric lenses in contacts that can have beficial or detrimetal effects on vision. Depends on the distance from eye, degree of asphericity (I know in lectures, speakers like to say: "It's aspheric and you are supposed shut up then and act like he knows more than you, but often he doesn't even have the concept, it's just trown in to make him look smart and you scared to ask questions.).
    For years we used this in low degrees in contact (concept David Volk) for fitting (he told me had not really conceived how peripheral curves were used when he came up with this). We also found that in some cases contrast was increased. Many bifocal contacts today have nothing but asphericity used to accomplish this.

    However back to your origional question, I think graph images are available on most aspheric spectacle lenses which should give you a good idea about how much or how little help asphericity would accomplish at given powers, thicknesses and vertex(s).

    Chip

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    Allen Weatherby
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    Aspheric a misunderstood term

    Quote Originally Posted by go_hercules View Post
    Hi, I have been wondering if there is an effective minimum diameter at which an aspheric lens loses it's advantage. The aspheric surface is most spherical near the center and the curvature gradually flattens out further from the center. So, if the diameter of the lens were say the size of a quarter, you would not be getting as much into the more aspheric zone than if the lens were a couple of inches in diameter. In other words, it seems to me that as the diameter of the lens increases, the usefullness of the aspheric lens increases, and vice versa. So is there a minimum diameter at which the benefit of asphericity is nil? I would be interested in others thoughts on this.
    The term Aspheric is not a lens design term in a technical term. It simply means, "NOT SPHERICAL". This industry adopted the term and in most cases opticians are referring to an optically functioning lens when using the word Aspheric. I think what you are asking will depend on the lens design. A progressive lens is Aspheric and they all have areas that are not designed for optical correctness, these areas are just fill in so the lens fits into the frame.

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