Aspheric glasses, blurry off center. Raise optical centers? (was:smaller glasses, blurry off center)

Discussion in 'Glasses' started by Fred Ma, Nov 2, 2004.

  1. Fred Ma

    Fred Ma Guest


    Hi,

    I'm still trying to resolve this off-axis blur. Let me recap on what has
    transpired since.

    Man-years old lenses
    --------------------
    * High index plastic, n=1.6
    * Spherical lenses
    * -3.25/-4.75 sph
    * Base curve +3/+4
    * Axis correction: unknown

    First new lenses
    ----------------
    * High index plastic, n=1.67
    * Aspheric
    * -5.25/-4.00 sph
    * Base curve +2
    * Prescription's axis correction: 015, right lens
    * There was an error in the axis correction of the
    right lens, but that's not were problems were noticed
    * Problem experienced with left lens:
    Blur starting at 30 degrees left of center,
    quite pronounced starting at 45 degrees

    Second new lenses
    -----------------
    * Attempt to replicate parameters of old lenses
    * High index plastic, n=1.6
    * Spherical lenses
    * -5.25/4.00 sph (same as first new lenses)
    * Base curve +3 (from +1)
    (even though first new lenses measured at +2 above)
    * Peripheral blurring reduced, but still present
    * Center vision of left eye seemed poorer than with
    first new lenses, and poorer than right lens
    (not much, but noticable)
    - Lens power was confirmed to match the prescription
    * Local optician (different from where it was bought)
    adjusted glasses to raise them, further reducing
    peripheral blur
    * The improved position seemed a bit assymetric
    (is offset slightly to the right)

    Because of the improvement from adjusting the position of the glasses, I
    decided to give the first new lenses another try. I was hoping that it
    will almost eliminate peripheral blurring, as asphericity was meant to do.
    After putting the aspheric lenses (first new lenses) back into the frame, I
    still found the peripheral blur very pronounced, much more than what I
    thought reasonable for one to "adapt" to. After a few days of trying to
    adapt, I found that I could noticably reduce peripheral blur by manually
    holding the frames in a particular way.

    Manually positioning aspheric lenses for reduced peripheral blur
    ----------------------------------------------------------------
    * Horizontal position is symetric again
    * Vertically higher up so that I stared
    out the center of the lenses

    I didn't actually notice that I was staring out the center until I visited
    another local optician to adjust the fit. He suspects that the optical
    centers of the glasses are centered with respect to the frame, at a lower
    "latitude" than my pupils when staring straight ahead. He said that this
    is standard for spherical lenses, and it also falls quite in line with
    Robert Martellaro's description of proper fit (top). However, he has heard
    that for aspherics, it can be better to raise the optical centers
    (apparently as it is done for "progressives") to the same level as the
    pupil, which is typically above the center latitude of the frames.
    I confirmed that when I stared out the optical centers, my ability to
    read clearly is not compromised when I look downward.

    I talked to a technician at another optical retail establishment, and she
    confirmed that for lenses of nontrivial strength, she automatically raises
    the optical centers regardless of whether they are spherical or aspheric.
    Her impression was that this was a common practice to avoid the more
    pronounced distortions of higher powered lenses. She starts to consider
    raising the optical centers for strengths of 2 (I suspect she means -2.00
    sph, in the case of short-sightedness).

    How common is this practice of raising the optical centers for high
    powered prescriptions? What is typical? I stare out the line
    delineating the top third of my frames, but this could even get as
    high as the top quarter depending on the time between pushing up of
    my glasses. The greater the interval between pushing up, the more
    the glasses can slide down (and any amount of adjustment doesn't
    seem to prevent this). What is the typical amount to raise the
    optical centers, and how is it determined, considering the sliding
    down of the glasses?

    Sliding down of the glasses exacerbates the determination of how much to
    raise the optical centers, and I suspect this is strongly influenced by
    face type. As an asian, my nose is flatter, which probably worsens the
    sliding. I suspect for nonasians with more protruding noses, the nose
    structure gives better resting surface for the nose piece and keeps the
    glasses up better. The sliding down of glasses for asians might be partly
    why there is such variability in the benefit experienced with aspheric
    lenses. The aspheric shape is based on very specific positioning between
    glasses and eyes; for people with flatter noses, this is not easily
    maintained.

    Any comments welcome e.g. on:
    * The likelihood that the optical centers are indeed centered to
    the frame, below the pupils
    * The advisability of requesting lenses with raised optical centers
    (whether it might cause more grief than good, due to some
    unanticipated side-effect)
    * Tricks for determining a "all-round" good position to raise them to
    * Whether there are specific racial trends that might affect how one
    might choose this position

    Fred
     
    Fred Ma, Nov 2, 2004
    #1
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  2. Fred Ma

    Fred Ma Guest


    Hi, Robert,

    Can you please point me to some basic information about fitting? I'm not
    sure what is involved in pre-adjusting the frame. I looked at
    http://www.harcourt-international.com/e-books/pdf/246.pdf (last section
    titled "Astigmatism of Oblique Incidence", and my naive way of picturing a
    tilted lens evokes that image. I'm obviously not right about this because
    it supposedly increases the effective power of the lens. Is this
    preferable to raising to OC? Why would that be?

    When you say principal axis of the lens, is that similar to optical center?
    I guess the eye's center of rotation is close to the pupil location when
    staring straight ahead. Am I right in assuming this?

    I'm not that familiar with the acronyms used in optometry. I wonder if you
    could elaborate on what is PD (it means photodiode in opto- electronics).
    Monocular means pertaining to a single eye.

    My old glasses have silicon-feeling pads, while the new frames seem to be
    hard plastic. The friction seems similar, though a bit better on the
    silicon-like ones. I suspect that it is largely helped by a properly
    shaped ear-piece to keep the frame against the face (and thus the nose,
    which is doing the job of holding up the frame). Unfortunately, my nose is
    kind of flat. Thus my wondering about a possible correlation between
    racially related flat-face structure and unluckiness with aspheric lenses.
    My new frames are quite light, so I can't blame the weight.

    Aspheric seem to be so sensitive to position that I really am pondering
    whether it is worth it, despite its better clarity. By that, I mean
    staring out the optical center, not peripheral clarity. I actually don't
    know whether it is any clearer at the perihpery because it's difficult to
    switch quickly between them to compare -- they both occupy the same frame
    which requires a half-hour drive out to my opticians to change.

    The better on-center clarity of the asphericals could be attributed to
    inaccuracy in in the sphericals with respect to the prescription, but I
    already had the sphericals checked. The only explanation I can think of is
    that there was an inaccuracy in the asphericals to give me better than
    20/20 vision, but those were also checked. It just seems that asphericals
    are just simply better on-center, period. If it wasn't for their
    positional sensitivity, asphericals would clearly be better all-round, in
    my case.

    Fred
     
    Fred Ma, Nov 2, 2004
    #2
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  3. Fred Ma

    Fred Ma Guest

    Just an addendum to positioning/fitting of frames as one degree of freedom
    to control/minimize peripheral blur. I found out just how much
    chromatic aberration plays a role too.

    I stared at white letters on black background. Pretty blurry peripheral
    vision. I did the same for green letters on black background. Much
    reduced peripheral blur. So Robert Martellaro's mention of transverse
    chromatic aberration has taken on a new reality for me. I spend lots
    of time in front of a computer screen looking at color-coded text,
    but unfortunately, not all the same color (like a kaleidascope).
    Also, unfortunately, the default in today's computer applications is
    to present information as black text on white background, so it
    really isn't an option to try an customize all work envrionments
    to use primarily one color.

    According to my optician, there isn't much in the way of alternatives
    in plastic. My current aspheric lesnes are "centoptic" (she wasn't
    sure whether it is the brand name of the material, the manufacturer,
    or the importer). However, Centoptics is a subsidiary of Centennial
    Optics. Google didn't come up with any hits on either. It's
    probalby not that important, though. The specs were still available:
    n=1.67, abbe=32, 1.1mm center thickness. Froma bit of web surfing,
    it seems that the abbe value is not out of normal for high index materials
    in that range of refractive index.

    Fred
     
    Fred Ma, Nov 2, 2004
    #3
  4. Fred Ma

    Fred Ma Guest

    Min lenses are medium-power single-vision lenses. I'm not sure of the
    bases on which I can assess an optician's competence. My optician was
    aware of the issue with optical centers being below the pupil, and that it
    is common practice to raise them for higher power lenses. Her choice for
    keeping them at the center of the frames was based on the fact that on my
    old glasses, they were a full centimeter below the pupils. Since the new
    frames are smaller, putting the OC at mid-frame effectively raises it
    toward my pupils. The rationale seemed reasonable. I'm not sure whether I
    would have made the call any differently, if I was an optician. Granted, I
    think much of the technical details could have been communicated to me as
    it was being done so that I can be more informed about how to go about
    experimenting for best clarity. But I don't expect many customers would
    want to delve into the technical details, so I may be an anomaly
    (relatively speaking). They have been pretty forthcoming with information
    and rationale when I explain to them what I have tried, and what my
    suspicions or questions are. Admittedly, though, I am frustrated that I've
    spent so much unaffordable time with it, but I don't think I would have
    been in a position to appreciate these details if they were dumped on me at
    the start. Part of the frustration is that they are situated so far away
    from me, but that's my own doing. I asked for recommendations on a local
    newsgroup, and that was the only one provided. I was not aware of how
    involved this can be.

    Since I've already bought the glasses, and they have already cut 2 pairs of
    lenses, I feel that reneg'ing on the purchase is only justified in the case
    of gross incompetence. Granted, the 2nd pair had to be cut due to an error
    in the first set, but they promptly made good on that. I can't say that I
    feel that there has been gross incompetence, though the staff might not be
    the most experienced with aspherics; the lab guy admitted that. The issue
    about communicating technical details up front, I'm still asking myself
    what is a reasonable default conduct, given that most of the population
    probably isn't interested in the technical details.

    I appreciate your suggestion, and I admit that I am quite anxious about this
    whole issue, since it is a huge chunk of change on a product that I hope
    to amortize over a number of years. I want to make the best decision.
    However, I don't want to be unfair or unreasonable (neither do I want to
    shoot myself on the foot).

    Fred
     
    Fred Ma, Nov 3, 2004
    #4
  5. Fred Ma

    Fred Ma Guest

    Well, that was rather naive. All the time I thought I was trying to make
    the situation work, I was actually becoming a nuisance. We have both
    agreed that it wasn't working, and parted (amicably, I'm happy to say).
    The bummer is that earlier the same day, the back-room technical expert
    spent a lot of time with me trying to find a solution, and it seemed like
    we had one. On one hand, I wanted to keep that hard-earned solution, but
    on the other hand, it would have left a long-lasting and bitter after-taste
    to deal with a front-line person to whose patience had run out. I thought
    that her reaction was ironic; it wouldn't be an exageration to say that I
    spent at least 10 times the amount of time they did trying to find a
    solution, driving an hour round trip each visit, researching the possible
    causes and fixes, getting independent verification of lens parameters, and
    trying to understand their position as an independent business.

    I think I worsened the situation by not recognizing it for what it was and
    bowing out earlier. In the end, the solution isn't worth it no matter how
    good or hard-earned. I didn't like the idea of possibly having to go back
    for adjustments, or even being reminded of the situation whenever I saw or
    thought about my glasses. However, on bowing out, I feel bad about the lab
    tech guy, who spent a lot of time getting me on board with the things that
    could be wrong, and how to go forward finding a solution. It would be
    great if I could have dealt with him directly, but that's not how the
    business was set up.

    I am now faced with the problem of having to re-acquire the solution. In
    order to avoid becoming an imposition for whatever place I go to, I want to
    try and understand the solution before asking for it. The solution is
    bizzarre, and defies understanding even for the lab-tech guy, except at a
    conjectural level. What happened was that 2 sets of lenses were made for
    me. The first set (aspherical) had a mistake in the right eye. In making
    the 2nd set (spherical), they also tried varying the parameters to
    eliminate the pronounced peripheral blurring in both eyes. I tried the
    set#1 for a number of days (upto a week), set#2 for a number of days, and
    set#1 again for a number of days. Each time I switched, I found out more
    about how to position my glasses to lessen the peripheral blurring, but it
    was always very present.

    I was quite ready keep set#2 because the peripheral blur was much less; the
    only problem was that the on-center vision was noticably worse on the left
    eye. The lab-tech guy was trying to find a cause and found no spherical
    aberrations on-center or off-center. Quite miraculously, when he popped
    the lens in the frame, I found that I could see perfectly through them,
    on-center and off-center. I couldn't believe it. Exactly the same lens
    and frame, no blur at all. He also gave me an uncut version of set#1, and
    when I held them in front of my eyes, I could see through them perfectly
    too, on-center and off-center. All this, despite the pronounced peripheral
    blur that I saw in both sets earlier. I was all ready to take set#2 as the
    final solution, since set#1 had an error in the right lens. The above
    misunderstanding (if I can call it that) occurred later that day.

    Why would lenses and frames that were so blurry over several days suddenly
    become perfectly clear? I can't blame the blurriness on distortions from
    the frame, since set#2 was in the frames when I found it to be crystal
    clear. The lab guy said he was pretty careful in cutting the lenses to
    avoid a bad fit with the frame, which would result in distortion from
    stress. I also saw the way he popped the lenses in and out of the frame,
    and there isn't much room for error there, no matter who had done it
    before.

    The lab guy says that people's eyeballs actually change physiologically (I
    can't remember if he said over hours or days). While I can see that might
    make a bit off difference, I find it hard to see how that can account for
    such a marked difference in clarity. Plus, I saw a blurry periphery in the
    set#1, in the frames, just minutes before seeing a clear periphery in uncut
    versions of set#1. Perhaps set#1 was improperly cut, and suffered from
    stress from frame. The difference in peripheral clarity was quite
    dramatic, however, and it would be quite an eye opener to know it can be
    attributed to deformation from the frame.

    I can only conjecture one explanation, and it only explains the new-found
    clarity of set#2. Prior to putting set#2 back in the frames, I had been
    trying very hard to adapt to set#1. Maybe this made my focusing muscles
    strong enough that when I looked out the periphery of of set#2, I could
    compensate for the stronger power. Recall that peripheral blurring was
    much worse in set#1 than set#2; my muscles might have gotten strong enough
    over the past week or so to unblur the periphery of set#2, but not for
    set#1. I can't attribute the improved clarity in set#2 to lighting
    conditions. I wore set#2 driving home, and did not experience the degraded
    on-center clarity that I did before on the left eye. This remained true
    when I looked out my apartment window at the same land marks that I did
    before.

    If any experts can comment on the possible explanation for such a sudden
    improvement in clarity in both lens sets, I would really appreciate hearing
    them. I don't want to repeat the situation where I'm getting the
    optician's staff to chase down a phantom problem, and becoming exasperated.

    Thanks,

    Fred
     
    Fred Ma, Nov 6, 2004
    #5
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