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

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

Robert Martellaro

2004-11-02, 7:12 pm

On 2 Nov 2004 05:28:49 GMT, Fred Ma <fma@doe.carleton.ca> wrote:

>Robert Martellaro wrote:
>
>
>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

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.

Hope this helps

Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
robopt@execpc.com
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman