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Thread: aspheric vs. non-aspheric focal lengths

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    aspheric vs. non-aspheric focal lengths

    Does anybody have any information or know of any research that shows differences in focal lengths between aspheric and non-aspheric lens designs. I had someone hypothesize that an aspheric lens will produce a slight change in focal length than a spherical lens with like parameters. Wondering if the peripheral change in curve on an aspheric can have any measureable effect on focal lengths.

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    What's up? drk's Avatar
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    Certainly "aspheric" (broad term) lenses can produce multiple focal lengths for a given lens: look at a progressive. Look at a soft aspheric multifocal contact.

    Now, I think you're talking about a SV aspheric ophthalmic lens. I think the SV aspheric could be designed to do that, but it would be better to avoid it!

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    ATO Member OPTIDONN's Avatar
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    Quote Originally Posted by epiclabs View Post
    Does anybody have any information or know of any research that shows differences in focal lengths between aspheric and non-aspheric lens designs. I had someone hypothesize that an aspheric lens will produce a slight change in focal length than a spherical lens with like parameters. Wondering if the peripheral change in curve on an aspheric can have any measureable effect on focal lengths.

    Asphericity would have no effect on a lenses focal length. The small central area is essentially spherical but as you move toward the periphery the curve changes.

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    Master OptiBoarder Darryl Meister's Avatar
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    Donn is correct. The central region of the lens produces a "paraxial" focus equal to the focus produced by a conventional single vision lens.
    Darryl J. Meister, ABOM

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    Yes, this is a single vision lens. Should've mentioned that. I've always assumed that the focal lengths would be equal as long as the reference point was fixed away from the aspheric zone. Having said that and having that ingrained in my head I didn't know how to respond when I heard this theory. The premise is that the convergence of light created peripherally somehow has a very, very minimal effect on the focal length. I would assume that the theory also depends on how drastic the change in curves and the size of the point of reference. Say for instance your central non-apheric zone was only 1 mm round and the asphericity started there. Is it possible that if your reference point is small enough that the outlying aspheric zone can have some effect? Any more thoughts?

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    ATO Member HarryChiling's Avatar
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    It would change the vergence in the periphery, but light passing through the optical center of a lens has no vergence. You could wear a -50D lens and see fine if you were looking throgh a pinhole. The periphery is where the aspherics come into play. A -2.00 Rx in best form would need to be made on a 5 Base lens to have minimal peripheral distortion, but if the lens employed aspherics to correct the periphery than the base curve options are limitless. You could use a 8 base, a 1 base as long as the aspherics correct the periphery.
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    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by epiclabs
    The premise is that the convergence of light created peripherally somehow has a very, very minimal effect on the focal length.
    In reality, it would have a minimal effect upon focal length. However, when looking through the central region of the lens, where the focal power of the lens is typically defined, the pupil of the eye limits the region of the lens sampled to an area roughly equal to the size of the pupil. Consequently, because of the pupil stop of the eye, the focal length through the paraxial region is not affect by peripheral rays.

    Quote Originally Posted by Harry
    It would change the vergence in the periphery, but light passing through the optical center of a lens has no vergence. You could wear a -50D lens and see fine if you were looking throgh a pinhole.
    Light passing through the optical center still has vergence. You may be thinking of a single ray passing through the optical center, though a ray is simply a geometrical concept; the only time you will ever see anything close to a single "ray" of light--as opposed to a bundle or beam--is under very sophisticated experimental conditions. A pinhole simply "stops down" the bundle of rays to the point that the blur circle created in the focal plane becomes relatively small (at least compared to the depth of focus of the eye or optical system).
    Darryl J. Meister, ABOM

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    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Darryl Meister
    Light passing through the optical center still has vergence. You may be thinking of a single ray passing through the optical center, though a ray is simply a geometrical concept; the only time you will ever see anything close to a single "ray" of light--as opposed to a bundle or beam--is under very sophisticated experimental conditions.
    Of course it sounds better with a silver tongue. A ray of light.... :cheers:
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