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Thread: how to calculate power away from the OC.

  1. #1
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    how to calculate power away from the OC.

    Apparently I just do not know how to ask this question correctly: How do I calculate power drop on a lens as you move away from the lens center?

  2. #2
    What's up? drk's Avatar
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    Who says power drops? Does power drop?

    I mean, say a "pencil" of light comes through the center...it will mostly focus, nicely.

    Say a "flashlight" of light comes through the whole lens...the peripheral light will focus too far forward, I guess, due to spherical aberration.

    And I guess chromatic aberration occurs.

    And all that Seidel aberration stuff.

    But what is "power drop"?

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    Quote Originally Posted by drk View Post
    Who says power drops? Does power drop?

    I mean, say a "pencil" of light comes through the center...it will mostly focus, nicely.

    Say a "flashlight" of light comes through the whole lens...the peripheral light will focus too far forward, I guess, due to spherical aberration.

    And I guess chromatic aberration occurs.

    And all that Seidel aberration stuff.

    But what is "power drop"?
    http://64.50.176.246/cecourse.php?url=lens_design/

    vision and lens design section

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    Ghost in the OptiMachine Quince's Avatar
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    This seems like it would follow the same math as for compensating (which I don't know but I do have a calculator! [made by the great Mr. Chilling])

    When we compensate an Rx to fit a -2.00 sph in an 8 base wrapped frame, they see as they would through the actual -2.00 Rx on the appropriate base (like a 3.) So even though the actual power of the lens reads accurate away from the focal point, it does not appear as such to the wearer because of angle of their gaze.

    I know this doesn't answer your question, but maybe it helps flesh it out?
    Have I told you today how much I hate poly?

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Are you wondering about how the distance from the eye the power is affected? That would be Pretiss Rule wher P= C x D divided by 10.

    Or on our home page scroll down to "OPTICAMPUS" then optical calculators and click on tools for plug in power compensations.

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    What's up? drk's Avatar
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    Vision and Lens Design

    Lens aberrations manifest themselves as departures from the desired prescription. For instance, the lens aberrations produced by "flattening" a lens form (i.e., using a base curve that is flatter than recommended) increase the spherical focal power perceived by the wearer in the periphery of the lenses and induce unwanted cylinder power (astigmatism). The result is a change in the effective power of the prescription away from the optical axis (or optical center) of the lens, leaving a "residual" refractive error.
    The errors from the desired prescription produced by these lens aberrations result in blurred vision in the periphery and a restricted field of clear vision. Consequently, an imprudently flattened lens design, while thinner and lighter in weight than a "best form" lens design, produces inferior peripheral vision. The best form lens design, on the other hand, offers a wide, clear field of vision.
    The prescription errors caused by lens aberrations will increase with the following factors:
    • Distance from the optical axis/center: The farther the wearer looks into the peripery of a lens, the greater the potential for lens aberrations—and the more rapidly those aberrations will increase.
    • Departure from best form design: The farther the lens form departs from the recommended "best form," the greater the potential for lens aberrations.
    • Strength of the prescription: The stronger (plus or minus) the focal power of the lens, the greater the potential for lens aberrations.

    To summarize, the goal of best form lens design is to determine the most "optically appropriate" base curve for a given focal power (or range of focal powers). This means selecting a base curve that will produce a lens form free from the lens aberrations that can blur vision through the periphery of the lens. This process is referred to as lens design or optimization. When the doctor prescribes a certain prescription, he/she is really specifying the focal power "on-axis." The focal power "off-axis," however, is ultimately controlled by the design of the finished lens.

  7. #7
    What's up? drk's Avatar
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    If you're referring to that, I have downloaded a optical calculator (somewhere) that doesn't let you do the math, but it lets you play with variables to see how much you are going to screw up the lens.

    Darryl made that, too, I think.

    He was the greatest. Evah.

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    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by drk View Post
    If you're referring to that, I have downloaded a optical calculator (somewhere) that doesn't let you do the math, but it lets you play with variables to see how much you are going to screw up the lens.

    Darryl made that, too, I think.

    He was the greatest. Evah.
    Darryl Meister was the king of ophthalmic optics- rubbing elbows with apprentice opticians as well as PhD's.

    Optical analysis is included with the program below.

    Spectacle Optics v2.0 Beta NEW VERSION!

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    Quote Originally Posted by drk View Post

    He was the greatest. Evah.
    Yes!

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    What's up? drk's Avatar
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    Robert's not a close second, but he's second, nonetheless!!! :)

    (Big gap to third place, though.)

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Quote Originally Posted by drk View Post
    Robert's not a close second, but he's second, nonetheless!!! :)

    (Big gap to third place, though.)
    My vote would go to a very close third Harry Chiling- but he doesn't suffer fools gladly so he rarely posts where I do.

  12. #12
    What's up? drk's Avatar
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    I thought you were third.

    Fezz/Johns tied for last.

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    Master OptiBoarder rbaker's Avatar
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    Other than an obscure academic interest why would anyone want to know the Dioptric power of a lens off the OC? Just wondering if you have a practical reason for determining this value.

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Quote Originally Posted by rbaker View Post
    Other than an obscure academic interest why would anyone want to know the Dioptric power of a lens off the OC? Just wondering if you have a practical reason for determining this value.
    Academic reasons only is right!

    We did a fair amount of this at OSF where we were given curvatures with x and y reference points and needed to calculate power at z. Same for thickness and Gauss equations for ray tracing focal points (ugh).

    The spiffy TI calculators we used were invaluable and made you wonder how these pioneers ever did this on paper. Can't say I remember much (ok--any!) now, but that was 40 years ago.

    Had to do mostly with the NY State exam at the time having those questions and they wanted us to be able to take and pass a test in any state.

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    Master OptiBoarder rbaker's Avatar
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    A few of us from WITI took the NY Exams in 1969. I claimed NY State residency using my uncles residence on City Island in Brooklyn. Never used the license but have it an a scrap book somewhere. I also recall buying a TI-30 for under $300.00 sometime later but too late for the NY Boards. As I recall the NY State Boards were the gold standard back I the day.

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    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Jaketull View Post
    Apparently I just do not know how to ask this question correctly: How do I calculate power drop on a lens as you move away from the lens center?
    You will need to perform a backward ray trace from the eye's center of rotation(stop distance). You will need to know the material, thickness, R1, and R2, vertex distance, center of rotation, and eye rotation.
    You should read Mo Jalie, "Principals of Ophthalmic Optics".

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by lensgrinder View Post
    You will need to perform a backward ray trace from the eye's center of rotation(stop distance). You will need to know the material, thickness, R1, and R2, vertex distance, center of rotation, and eye rotation.
    You should read Mo Jalie, "Principals of Ophthalmic Optics".
    Along with Coddington equations......Pass the aspirin....

  18. #18
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by optical24/7 View Post
    Along with Coddington equations......Pass the aspirin....
    I really enjoy sitting with pencil and paper and sketching a good ray trace. Instead of aspirin, maybe use beer, whiskey, wine, etc.

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