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

2004-11-08, 7:12 pm

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

2004-11-09, 7:09 am

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

2004-11-09, 7:12 pm

Robert Martellaro wrote:

>Fred,
>
>Pre-adjusting simply means aligning the frame in front of the eyes in an
>optimum manner and then measure for lens position. Although it's good
practice
>to do this on all fittings, this step can usually be skipped when fitting
>spherical low power single vision lenses.
>
>The next step is to start screening for an optician that has the
knowledge and
>skill to fit the type of lenses that you desire.

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