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Thread: High power lenses

  1. #1
    OptiBoard Apprentice Bill M's Avatar
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    Redhot Jumper

    We see alot of high power (above +/- 10.00) orders. For those customers with the high minus Rx, there are several choices but don't seem to be many for the high plus. Is there a logical reason the lens manufacturers don't make a high index lens above a 7.00 base?

    As long as I have you thinking, how strong of an Rx should you use a PAL?

    Thanks

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    Master OptiBoarder Clive Noble's Avatar
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    Thumbs up

    Originally posted by Bill M:
    We see alot of high power (above +/- 10.00) orders. For those customers with the high minus Rx, there are several choices but don't seem to be many for the high plus. Is there a logical reason the lens manufacturers don't make a high index lens above a 7.00 base?

    As long as I have you thinking, how strong of an Rx should you use a PAL?

    Thanks
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    We also see a lot of high power prescriptions and have been sucessful with up to a +12.00 (if I remember correctly)using Optima Hyperview 1.66 which was made in bi-convex form.

    The problem was explained to the px but he insisted he wanted it, and it has worked.
    I believe it's mind over matter!!

    There's also another old PAL from Essilor, but only in Europe, called the Varilux Omega, a design like the SuperMod and again, over the years have had great success with this lens for aphakics

  3. #3
    Master OptiBoarder Jeff Trail's Avatar
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    Unhappy

    Originally posted by Bill M:
    We see alot of high power (above +/- 10.00) orders. For those customers with the high minus Rx, there are several choices but don't seem to be many for the high plus. Is there a logical reason the lens manufacturers don't make a high index lens above a 7.00 base?

    As long as I have you thinking, how strong of an Rx should you use a PAL?

    Thanks
    Bill,

    According to all the guys I know on the manufacturing side the main problem is in the molding. Those steep base curves play havoc with the process :)
    Originally the whole idea behind the "high index" was to help with the myop's but needless to say we all expect them to have every thing we ever needed when it comes to lens design.
    In the lab their are some tricks to help with the problem, I use the Kodak a lot (9.75 curve with an index of 1.57) in FT and also keep in mind what Clive was talking about, because if you go up in index you can get more power through less curve. I use the Optima choice also often. Also request that the lens always be cut on center (NO prism decentration) it will make for a thinner lens over all (profile)
    As for PAL's.. well peripheral distortion and oblique powers (astgmatism)is already not the best in the world in that type of design, now add the factor having to compound that problem with the higher degree of power.. yikes..
    I have cut the convex mixture but usually don't recommend it if at all possible because of the decrease in optical quality but some patients don't want to hear about that :) I know that I can get up to a +10.50 on the Kodak and still keep it with a convex ocular curve (did a pair about two days ago)Also if you can find the right shape frame and style then going with a super modular ashperic isn't all that bad as long as you can keep the bowl from getting edged into.
    The thing I always "pound" into the opticians I deal with (either through the lab or used to through my retail sights) is that these guys already know their eyes are not all that great and they KNOW that it isn't going to look like a pair of demo lens in the frame :) so get familiar with how the optics is going to relate to the frame choice and use that to make the difference.
    I think they have it down fairly well and it's always handy to have a "demo" of a ASM on hand to make sure if it can be done with the PD and stay away from cutting into the bowl. That only took me "years" of trying.. as well as now probably I have about 95% of my accounts into the habit of checking PAL fittings on the charts before ordering.. I even supplied them all the charts so it takes only a second to give the demo lens a little dot and then lay it down on the chart to see if it will cut out.. the vast majority of them love doing it now since it's easy to SHOW as well as explain to the patient exactly what they are doing and why it will not work.. sure beats me having to call the optician and tell them this PD, design and frame are not going to work..then call the patient back in and explain it.. helped improve the staffs abilities as well as made my life a lot easier :)

    Jeff" always with an opinion..be it right or wrong" T. :)


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    Confused

    The high curves are a nightmare. Coating them is a nightmare, too.

    The worst factor, however, is cost. Imagine how much the molds, R & D, and spoilage on those base curves cost. Then imagine how few times a year you will order those lenses. Finally, picture the taxes paid on all that lens inventory at the warehouse.

    Brad "It can't cost more than it makes" Gelb

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    Master OptiBoarder Jeff Trail's Avatar
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    Confused

    You guys might want to check out some lens from Signet Armorlite, I have a +14.25-1.75 that I'm cutting and I'm getting it with SA's HyperAspheric 15 base...it's a ft22 but it works pretty well and cutting it on center does make it as thin as you're going to get with that type of power..
    The HyperAspheric is available in SV,RD22 & FT22 base curves of 10/12/13.50/15 upto a 5 add :) of course the blank size is only a 65 mm but then again who would want that much power in a larger frame?

    Jeff "grinding away at sand..making my own lens maybe?" T.

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
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    I would say that are a number of reasons why you don't see many high base lenses. Some of them have been pointed out in earlier postings, but I will provide my own perspective on it:

    1. Manufacturing challenges: As mentioned in an earlier posting, there are certain manufacturing difficulties inherent in high base lens blanks as a result of their steep curves and unequal blank thicknesses.

    2. Rx range: A 0.50 base blank might be made to cover lens powers below -7.00 D. This single base curve though, can be used for just about ANY minus power below -7.00 D, even up to -20.00 D (if the thickness allows). There is really no lower (minus) optical limit to what you can put on that base blank. A steep base curve, on the other hand, is limited optically. For instance, you would not want to put anything higher than maybe a +7.00 or +8.00 D on a 10.00 base blank. Beyond that power, the peripheral optics will be poor and the back surface will approach a flat to convex curve. Also, beyond a +8.00 D power, the lens really has to be made aspheric in order to provide good peripheral optics.

    3. Demand: Frankly, there really isn't much of a demand for very steep base curves (for higher plus powers). High plus and minus powers represent very small percentages of the overall Rx market. There really aren't THAT many people walking around with > +5.00 D lens Rx's. Since there isn't as high of a demand, lens manufacturers might put their steeper base blank developments (and flatter ones, as well) on a lower priority list when introducing new lens designs in order to keep costs down to the dispenser and be able to bring new products to the marketplace faster.

    Best regards,
    Darryl

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