Vision - Aspheric glasses, blurry off center. Raise optical centers? (was:

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

Fred Ma

2004-11-04, 2:10 am

Robert Martellaro wrote:
>
> First check the fit. Have an optician mark the optical centers
> horizontally and vertically. The OC should be on the center of the pupil
> for the horizontal and 2-5mm below the pupil on the distance gaze, with
> the lenses almost hitting the eyelashes. If the fit is ok, and the
> blurring is unacceptable, replace the lenses with a higher abbe value
> material. Seiko makes a nice 1.6 index aspheric lens that would almost
> be as thin as the 1.67 but with better off-axis optics.

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

2004-11-05, 10:08 pm

Fred Ma wrote:

> Robert Martellaro wrote:
knowledge and[vbcol=seagreen]
>
> <...snip...>
>
> 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).

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

2004-11-08, 11:09 am

Robert Martellaro wrote:
>
> Increasing the panto (lens tilt) will help if the OCs are too low. This
> will also make the floor look flatter if the panto was insufficient.
>
> The right way to do this is to pre-adjust the frame, then measure for the
> proper vertical OC position.
>
> Martin's Rule says that the lenses should be fitted so that the principal
> axis of the lens passes through the eye's center of rotation. This means
> taking a monocular PD, with the vertical optical center dropped 1 mm
> for every 2 degrees of pantoscopic tilt.
>
> To keep the lenses stable in front of the eyes use lightweight frame and
> lens materials along with adjustable silicon pads.

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.c...oks/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