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Thread: Seg Drops Keep Fallin' On My Head

  1. #1
    What's up? drk's Avatar
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    Confused Seg Drops Keep Fallin' On My Head

    Hey, I'm not sure I invented this, but I don't think in terms of seg heights with segmented multifocals, I think in terms of seg "drop".

    I originally thought of this to keep my newly-trained wife/optician's head clear: just dot everone's pupil center and take split pd's with the pupillometer, and we'll work out the details later. (I've since been fired as a boss, by the way:) )

    For MRP's and progressives, this is all we needed, so I empirically determined that a normal BF seg line is 8mm below pupil center, and a TF is 4mm. Either one can be manipulated 2 mm higher or 2 mm lower, depending on need (submitted for your amusement):

    E.g.
    BF standard drop 8mm
    BF for reading mostly 6mm
    BF for distance mostly 10mm
    (BF for sunglasses 12 mm)

    TF standard drop 4mm
    TF for distance mostly 6mm
    TF for near mostly 2mm
    (computer TF 0mm)

    What this has done, other than simplifying things, has also given me the ability to do this: If a segmented multifocal is being fit into a small frame, and you are worried about cutting out too much seg, if you look the dot on the lens like a fitting height for a progressive, the same values apply for optimal fitting height. 18 mm is pushing it, below 18 is no good, 25 is nice. A FT-28 seg's vertical dimension is 20 mm, so fitting it a 8 below gives you the full vertical dimension at 28 mm high, if you follow me. It's halfway cut out at 18mm high, and 3/4 at 25.

    I thought these numbers would give me some sort of a standard to compare progressive designs to. While true corridor length may be difficult to ascertain due to different definitions, the theoretical corridor length of about 8 mm would put the reading area in the traditional position.

    But when I take out my ruler and lay it on the progressive lens identifier charts, from the fitting cross to the top of the near power circle is always 15mm, regardless of brand!

    I'm confused! Why are the circles so low, and why don't they vary by manufacturer? I would expect them to be close to 8mm below the fitting cross.

    Help me out, here, please.

  2. #2
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    I guess that when they start dispensing glasses from vending machines that this way of measuring would be sufficient. Everyone has different needs and segs will not always be in the same position. So it's probably a good thing you got fired. Take a little time and teach her the correct way to fit a pair of glasses. I just fit a pair of bifocals for a lady and measured center pupil to top of seg and it is 10mm.Terry

  3. #3
    What's up? drk's Avatar
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    Hey, I am a glasses-vending machine!

    Why did you fit her at 10mm and not 8mm, or 12mm? Probably because that's the way she was fit before, and before, and before. No problem with that. I call that "low standard", but if she's the world's biggest reader, you should have put them higher, right?

    I am individualizing! Just from a standard starting point using 2mm increments. Why diss me? Why didn't you use a 10.5 mm seg height, hmm?

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    My point is very clear then! There is no standard!

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    Quote Originally Posted by drk

    For MRP's and progressives, this is all we needed, so I empirically determined that a normal BF seg line is 8mm below pupil center.
    Surely as the seg tends to be fitted at the bottom of the iris, and the normal diameter is 9mm then the max below pupil centre you would want to fit a bifocal would be 5mm.
    That said i tend to fit round segs a little higher than flat top, and patients needs often differ from one to the next so there is no real hard and fast rule.

  6. #6
    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Before you get too excited, this is the definition of segment drop from the Dictionary of Ophthalmic Optics:

    "The manufacturer's specification for the vertical distance from the horizontal lens blank bisector to the horizontal tangent at the top of the segment (or the highest point of a curved top segment) on a semi-finished, ophthalmic multifocal lens blank supplied by the manufacturer."

    Unfortunately, if you start defining things in your own way, it can lead to real confusion when trying to communicate with others in the same industry, for instance the folks at the lab who may be trying to understand what you want fabricated. Every customer is different and even their needs will change with changes in their environments.

  7. #7
    What's up? drk's Avatar
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    Quote Originally Posted by rsandr
    Surely as the seg tends to be fitted at the bottom of the iris, and the normal diameter is 9mm then the max below pupil centre you would want to fit a bifocal would be 5mm.
    That said i tend to fit round segs a little higher than flat top, and patients needs often differ from one to the next so there is no real hard and fast rule.
    I agree with your anatomical reference point (we learned lower lid margin, but it's mostly tangential with the iris, but lids do vary while irides don't, so yours is better), but the visible iris diameter is, and you can research this, on average 10.6 mm, say 11.0 mm, so a 6 mm drop is more like it. I don't think that eight is too low, though, but your argument has validity. If you want to adjust my system by 2mm, no big deal.

    I'm doing this to formalize segment fitting. Standards are set to be personalized, but I don't think it renders the standards useless.

    If there are no standards, CME4SPECS, why didn't you fit her at pupil center, then? Shouldn't matter, if there is no valid standard!

    Judy, after saying all this nice stuff about you, you certainly don't think I'm so stupid as to call in my jobs with my own specification system, do you? I do the math and order the proper seg height the conventional way. I'm about standardization, not anarchy!
    Last edited by drk; 05-06-2004 at 05:05 PM.

  8. #8
    Quote Originally Posted by drk
    Judy, after saying all this nice stuff about you, you certainly don't think I'm so stupid as to call in my jobs with my own specification system, do you? I do the math and order the proper seg height the conventional way. I'm about standardization, not anarchy!
    drk=:angry:

    Listen Doctor. Ahem. There is no standard w/ respect to drop from pupil.

    A professional optician would do the lid as this thread references by reason of professional certification and training. I understand "being different" (as I am the epitomy of), but I think you are taking this one too far.

    I guess there will be more standards as soon as there are more standard patients!:hammer:

  9. #9
    One eye sees, the other feels OptiBoard Silver Supporter
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    "I originally thought of this to keep my newly-trained wife/optician's head clear"

    She's probably half way to the tavern by now.

    The signpost up ahead-your next stop, the Divorce Zone.

    R. Serling

  10. #10
    What's up? drk's Avatar
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    No more revolutionary ideas, I promise. Status quo all the way!:hammer:

    OK, I couldn't resist:
    There is a standard for drop from the pupil: FOR PROGRESSIVES! Nyah-nyah-nyah!:D

  11. #11
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    DRK,



    Hey! I'm with you !!

    For years it was difficult to retrain newly hired opticians as to our methods of measuring. For men. we measure all segs from the belt buckle up, using a tape measure. (for the Amish, we measure from where their suspenders attatch) Many folks have commented how we seem to really spend extra time on this. For women, we measure from their naval, or where we think it is, up. We used to measure the drop method, which was from the eyebrow, but then we had an influx of ethnic folks move to the area, and we weren't sure whether to measure from the top of the brow, the middle, or the bottom, as this can be a difference of 10-15".

    For PD's we stick a #2 pencil in each ear, (Eraser side in), measure from the points, and subtract 17mm. This is much more fun than the pupliometer, it lets you really build a relationship w/ the patient, and again, your doing something just a little bit different than the guy down the street.

    Here's to diversity!!

  12. #12
    What's up? drk's Avatar
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    Angry

    Har har har!

    Please tell me: what is the optical significance of the lower lid? Did God put our lower lids at a perfect seg height?

    Doesn't it seem more likely that the location of the segment would be better referenced to the section of the eye that we actually see through, the optic axis, which approximates the pupil center? No...we like the lower lid just fine.

    You opticians are a rough group. Rough. I am paying the price for trampling on a sacred tradition in your culture. My apologies.

    "Hey, have you heard about the new Essilor Blephara?! It's a free-form progressive individualized to each individual's lower lid position. The patient looks into a complex apparatus that measures the position of the lower eyelid in different positions of downgaze by the use of an ingenious tong, not unlike a lady's eyelash curler..."

    P.S. The aforementioned "optician" is long-since a full-time mommy. In her place is a well-paid, dual-licensed, 20-yr veteran that you would be well-pleased with. And she's almost as good as my wife was.
    Last edited by drk; 05-06-2004 at 11:27 PM.

  13. #13
    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Of course you're not stupid. That's not why I included that definition. Every profession has its own lexicon and for the sake of uniformity in communication we need to stick to it pretty closely. So, if you're the Doc and it's your practice and you're doing the training, then to someone who doesn't know any different, your way and your definitions are gospel. I just don't think we should muddy the waters even more than they already are. :)

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    drk

    The lower lid is just a starting point, just as 8mm below the pupil is. Have your wife try this...I find it pretty fool proof. If you like the 8mm below pupil that's fine for a starting point. Take a small post-it note and place it at 8mm below the pupil on both lenses, (you'll need to trim it a little so it doesn't go below the lens) the straight edge should be fairly straight. This represents where the top of the seg will be. Have the patient stand up (very important) and let them see for themselves where you intend to put the seg. If the distance vision is ubstructed by the post-it note simply move it down until the patient can see clearly over the top of it. Next have them sit down and give them a reading card to look at. If they can't see the reading card when looking down in a natural manner...perfect your done. Measure the top of the post-it note to the bottom of the lens and ship it off to the lab! Now you didn't decide where it goes, they did. I never have any trouble with this method. Now we aren't such a rough bunch now are we? Terry

  15. #15
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    DRK,

    Sorry for the sarcasm but I couldn't resist.

    As far as the lower lid is concerned, I must be missing something. I took at least 25 seg hts today, and I didn't notice where anyone's upper or lower lid was. It has no bearing on where I put the seg. I put one guy (a lumber grader) in a VLx CMFT lens that was 18 high in a frame that had a B meas. of 49. He won't be able to read the newpaper at the kitchen table, but they'll work fine for work. Again, I have no idea where his lids are. What about old follks who have lids that are dragging in their laps ?

    CME4Specs hit it right on: The customer tells me where to put the seg when I ask the right questions.

  16. #16
    What's up? drk's Avatar
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    I think I started a problem that I didn't mean to start. I was more interested in the question about the distance a person looks down through a progressive corridor as it relates to a typical distance a person looks down for a segment. Due to my unorthodoxy, I am conscious of the fact that most people look down 6-10 mm to read through their segs, and then they get however much vertical near area, depending on the B dimension, but usually at least 10 mm (half of a FT-28).

    Compare that to a progressive. If a person goes to a progressive, they are looking down further, because, if I'm not mistaken, corridor length (however defined) is about 14mm in most designs, and with fitting heights typically around 20-22, they get a full add vertical dimension of only 6-8 mm.

    Now I realize that this is very "progressives 101" and that, duh, this is the way it has always been, and that due to the astonishing success of progressives, maybe one doesn't need all that FT-28 room for general purpose reading.
    Of course, when you're looking through a progressive, the top of the page may be through the corridor while the center of the page may be through the full add which may help the size limitations.

    I do think it's interesting to conceptualize the differences in terms of hard numbers. It's made me appreciate segmented lenses a little bit more, but I doubt I'll sell more of them.

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