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Thread: Degressives and frame choice

  1. #1
    OptiBoard Novice OptiBoard Silver Supporter
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    Confused Degressives and frame choice

    Hi all,

    How different are the iso-astigmatism plots between degressive and progressives?

    And given that the reference power in degressives is the combined near add - is there more control over the lens during casting and cutting - so that having a shallower or deeper frame is irrelevant?

    I guess, my question in another form is: all else equal, if there are two frames (one deeper, one shallower) - would there be power difference at datum (usually the point of intermediate distance) and bottom (reading distance) on both frames?

    I currently have a px who is struggling with a deeper frame on the same Rx (different design) than her older, shallower ones.

    Any ideas?

  2. #2
    What's up? drk's Avatar
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    I think I know what you're talking about when you say "degressives" but that's not a commonly used term in the U.S.

    The only lens I know that is accuately described as degressive is the Sola Access (Synchrony Access). I think all the more common designs are called "near variable focus", and really that's been going out of style for the term "computer progressives" or "occupational progressives" etc. Just terminology?

    I think you're asking whether a given design has variable length from the fitting height to the near zone (or 85% of the near zone or whatever standard is used). That is a good question, and one I have not thought of.

    I would guess "no"--the rate of progression/degression is the same regardless of fitting height or B-dimension. I can't think of an advantage to a variable progression.

    Others?

  3. #3
    One eye sees, the other feels. OptiBoard Gold Supporter
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    Most PALs use variable corridor designs, primarily to assure that the near zone is fully utilized in the narrower frames. We can do this ourselves by ordering fixed designs, but then you have to think it through instead of pushing a button. As your example illustrates, because the software engine is mindless, it can only spit out solutions with no respect to the needs of each individual.

    Thus, your client's narrower frame simply had a shorter corridor than the deeper frame, and they adapted and/or needed it that way. The stupid engine made the corridor longer in the deeper frame, and/or the corridor length limit might be too long making this a poor design. Moral: use ophthalmic lens companies that employ vision scientists and perform wearer trials, selecting designs where the ophthalmic optician has more control over the lens design.

    WRT cylinder plots, if it is a true computer PAL, the add will typically be +0.75 to +1.50, resulting in much less unwanted surface astigmatism with wider spaced plot lines. If it's some Frankenstein-like jack of all trades 'office' lens, it will look like a plate of spaghetti (see image below), similar to fitting a general purpose PAL 8mm above the pupil.

    Hope this helps,

    Robert Martellaro
    Attached Thumbnails Attached Thumbnails Office Astig plot +2.50.png  
    Roberts Optical Ltd.
    Wauwatosa Wi.
    www.roberts-optical.com
    ~~~~~~~~~~~~~~~~~~
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    It is easier to fool a man, than to convince a man he has been fooled. -Mark Twain

  4. #4
    Ghost in the OptiMachine Quince's Avatar
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    My office always used to be in favor of short corridors and that mentality worked for us for years. Recently we have switch from Hoya PALs to Shamir and took our reps advice to heart. He suggested that we give the maximum corridor length possible because that is how Shamir designed their lenses to be worn. For a while we were still ordering with set lengths, putting high segs in mediums corridor lengths but have shifted even further to ordering all jobs as a variable length (unless circumstances dictate otherwise.) This has been extremely successful, with a decrease in redo rate for changing lens design.

    My correlation here, is that most computer lenses don't have corridor options and therefore are set at a variable, as stated above, meaning this is a comfort of wear issue. Because there is no option to specify length, you must do so with the frame choice. Kind of a bummer, but an easy fix.

    In my experience, people are much more particular about the fit of an occupational pair vs everyday pair. Which makes me wonder why we can't specify corridor lengths? The reason Shamir is against shifting the length unnecessarily is that more edge distortion is created (and here we see a potential tangent of hard vs soft designs.) So does that translate as the reason for not offering a shortened version? In dealing with a lesser variable strength but also precise positioning, often physically measured for the doctor adjust those strengths to, would specifying length be asking for more complications?

    ...

    Like, what?

    Sorry, I think I may have just confused myself.
    Have I told you today how much I hate poly?

  5. #5
    What's up? drk's Avatar
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    Mr. Controversy is here.

    Guys, I don't think the general concept of "corridor" applies to NVF (forgive the terminology) or occupational progressives, in general.

    [An exception: spaghetti "indoor progressives" (<--love that idea) also known as "office lenses that have a farther out focal length in the upper area of the lens". Because you will be looking through a corridor to see the computer.]

    I think most occupational PALs are set so close (like, 50-60 cm) that the difference between straight-ahead gaze (the computer monitor) and downgaze is a very small progression. Like, the add is going to be a diopter. With a mere diopter of add, and with the intermediate zone already taken care of, the "corridor" is probabaly just a "connector" or "umbilicus" that you race through on your way to downgaze. It's probably not even used.

    Now, I guess I could worry about the manufacturer's choice during the design process as to how much eye depression is needed to go from straight ahead to downgaze, but I assume they have their preferred angles built in.

    To sum, I don't worry about it.

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