Smaller glasses blurry off center

I recently got a new pair of glasses. My old ones are many
years old, and as was normal at the time, a lot bigger than
what is common today. The newer ones are significantly
smaller. I find that when I look at objects significantly
off of center, it is very blurry. The blurriness starts
around 45 degrees off of center. This is really disappointing.
I've seen pictures of what things look like from the point of
view of someone suffering from eye disease, and it kind of
looks like that (clear in the center, blurry off of center).
Probably no as severe, of course. But I'm not sure it is
something that I want to get use to.

The technician says that it is a natural consequence of the
smaller glasses, as well as edge distortion from the high
index glass. She says that the blurry off-centerness would
not be so severe for low index glass.

Is this in fact the case? Is what I described pretty typical?

Fred

 

Sort of what I thought. I don't have a background in optometry,
but I have studied optics from an engineering perspective.

I'll give details for the left lens, which is the one with the peripheral
blurring. According to the optician, the old glasses have -4.75sph (I
think it stands for "sphere") and the new glasses have -5.75sph. There
are also spaces on the prescription for "cylinder", "axis", "prism", and
"add", but they are empty for the left lens. All the lenses are single
vision. The new glasses actually use high index plastic, 1.67.

They claimed that the old one is also high index plastic, maybe 1.6.
Funny thing is that I was under the impression that I bought it as
high index glass, many many years ago -- with AR coating, though the
current optician says it doesn't have any. That's a bit of a bummer,
because the motivating factor for me to get new glasses is because I
scratched the old ones in the center of view. This might not have
happened if it has AR coating, since it also acts as a scratch-resistant
coating.

Anyway, the base curve of the new glasses is +2.00. On the old glasses,
it is +4.00. I also had the old glasses measured elsewhere, which gave
a base curve of +3.25. I'm not sure whether the discrepancy between
+3.25 and +4.00 for the old glasses is significant.

That other place also found that the new glasses has "spherical
aberration" in the area where I saw peripheral blurring. According to
my optician, though, this has limited meaning in a retail setting, where
most places do not have the means to measure it. It also wasn't clear to
her what the other place meant by spherical aberration, though admittedly,
a technical, unambiguous definition exists. My optician's lab guy also
claims that all lenses have spherical aberration.

Yet another discrepancy came up regarding info on my new glasses. I had
requested high index glass. According to my optometrist, it was
actually made with high-index plastic. When I brought it elsewhere to
check for problems, they claimed that it was definitely glass. When I
brought it back to my optician, she was adamant that it was high index
plastic. She explained that they decided to use plastic because it was
safer, cheaper, and pretty high index.

I should add that the new glasses were apparently aspherical, while the
older ones were not aspherical ("spherical"?). The strategy to correct
the peripheral blurring is to make the new glasses with as similar
characterisitcs to the old glasses as possible, including the use of
similar index plastic, spherical design, and similar front curvature.
They explained that aspherical is better, but was meant to be customized
for each client (I guess that means material, prescription, etc..). On
the other hand, commercial processes use a generic method in the
aspherical design, which can give undesired results.

Fred

 

Hi Fred,

Given the info it is possible, that being a high index plastic lens in a
shallow frame, that the problem is that the lens surface is being distorted
by the frame. I have seen this several times. The lens is secured by the
nasal and temporal edges of the lens but is not secured by the upper and
lower edges and therefore the lens twists in the frame.
On checking the lens powers on a lensmeter(focimeter) they are correct.
However using a radius gauge for checking the lens curves they are not
correct. The way to check this is measure the curves with the lenses in the
frame and then take the lenses out and measure the curves after about 30mins
if the curves have changed then distortion is present.

See if this helps

Regards


Ian Hodgson - Isle o

 

Hi, Ian,

I will ask about that. Just wanted to clarify what you meant by a shallow
frame. Do you mean that the rim is very planar, or that they are not very
tall? If you mean planar, would the rationale be that the lens curves, so
the rim must curve just to maintain contact at the edges?

Fred

 

I just asked the optician about this. She said that they do keep an eye out
for that kind of thing, and it shows up quite noticably when they try to
clamp the rim together and screw it together. Thanks.

Fred

 

Hi, Rob,

My experience with optics deals pretty well with numerical simulation of EM
fields along wave guides. Much if imaging optics is outside of my experience.
Would the above information be something that the optician and her lab guy
knows about, or is it worthwhile trying to convey this to them by phone? They
are quite a distance from me, so I don't think I'll be printing this out to
show them.

Fred

 

Hi Fred,
By a shallow frame I mean depth of top to bottme rim of 35mm or less. What
happens is that when the lens is edged in a lowish powered (say less then
about 5D in the higher powered meridian) then the resultant edge of the lens
is so thin it doesn't locate and secure in the rim groove.

Not to a later post unless they stress test the lens or use a radius gauge
it does not show up readily on glazing.

Regards



Ian Hodgson - Isle of Man

 

Hi, Ian,

Thanks for clarifying. Since I don't have the frames on me right now (they
are being fitted with new lenses), I don't know the exact height. From
memory, it is quite conceivable that they are are less than 35mm height.
That seems to be the modern style these days.

I'll print out the details you provided to see how the optician takes it.
I don't have the expertise and equipment to test it myself.

There are a few things which hopefully indicate that this is not the problem.
One is that my prescription is quite strong, so the lens is on the thick side
(-5.75sph). This is probably outside of your "lowish powered" ballpark, right?
As well, it is high index plastic, but at the low end of the high
index materials (I think it's 1.7); I am told that there are even higher index
materials. Another thing is that it occurs on one eye, the one with a stronger
prescription, but not the right eye.

I re-read your details and it seems to me that there are two effects that you
could be referring to. One is that lense is secured on only two opposing
edges, out of 4 possible edges. The lens is then able to shift position
because of this. This seems to fit your description of the edge of the lens
not fitting securely into the groove of the rim, particularly if the lens is
thin. The free play allows the lens to shift around. I'll have to take a look
at the final product (when I have access to it) to be able to relate the this
shifting. On my old glasses, the lens material forms a ridge around the edge
of the lens, apparently shaped to fit into the rim of the frames. When the
screws on the outer edge of the rim is tightened, the entire rim of the frame
wraps around the entire rim of the lens. Because of the ridged protrusion from
the edge of the lens fits into the groove in the frame's rim, there are are no
isolated anchoring points. The entire rim anchors the lens. I will look for
this in the new lens.

The other effect that I imagine based on your description is that the
edge of the lens doesn't align well with the rim, possibly brought on
by the fact that the upward force on the rim is only from the nose
piece. The metal arm that hooks around the ear doesn't provide any
force against gravity, it just prevents the rim from tilting forward
and falling off the face. In fact, the gravity of those arms might
provide downward force on the rim. Together with the upward force by
the nose piece, this creates a sheer force on the rim, which is then
transmitted to the lens, thus distorting its shape. I'll see if they
can do the test you described, checking the shape of the lense in the
frame, ideally after it's been on my face for a while, and then 30
minutes after the lens are removed from the frame. I'm not sure if
they will do this, but hopefully, they will see your reasons after
reading your description.

Thanks again for the explanation and suggestion.

Fred

 

Well, I drove out to pick up the corrected glasses today. It's certainly
thicker and heavier on the higher-power left side. Seems to want to weigh
down on the face more on the left side. The combination of
higher base curve of 3 (from 1), lower index of 1.6 (from 1.67), and
spherical lens shape seems to lessen the blurring at 45 degress to the
side, but there is still some. The lab guy referred to this as "obliquely
induced astigmatism", or "induced oblique astigmatism". He says it's
unavoidable, and suggested I google it. I can't seem to find much in the
way of such an effect for glasses, though it seems to seems be a common
thing problem for the eyeball itself, as well as a common side effect of
eye surgery. The lab guy's conclusion was that a higher base curve lessens
the off-center blurring because the lens curves around the eye (like a
panoramic). However, it yields a thicker lens. So functionally, a base
curve of 5 would be ideal, but cosmetically the pits. They
chose to compromise at 3, same as my old glasses.

I wonder if the extra thickness of higher base curve (say, 5) could be
offset by using the higher refractive index 1.67. Granted, the higher
index is suppose to worsen the off-axis blur, too, so the net gain in
terms of both blur reduction and lens thinness might very well be close
to zero. Could anyone comment on this, even if it's just speculation?

A friend of mine is convinced that the is something wrong when I explained
the blurring. She has glass prescription of -4.75 (same as my old ones,
before moving to this new -5.25) and says there is no blurring at all, in
any of the field of view within the frame of the glasses. She is also sure
of the same for some of her friends (though it's difficult to imagine
people actually talking about such details in normal daily conversation).
According to her, the switch to -5.25 is too minor to account for this
off-axis blurring. I think my optician's lab guy also hinted that the
increase wasn't that much. When I picked up my glasses today, however, I
asked my optician why I didn't experience such a degree of off-access
blurring with my old glasses, which has the same base curve, index, and
spherical shape. She attributed it to the big change in the prescription.
I wasn't sure how to interpret this, given the opinions that the change was
not that big. Is the change from -4.75 to -5.25 "big"? I don't want to
be making a fuss if the optician is right. I'm trying to be sensitive to
the fact that it is a small-ish business and probably not as able to
absorb the cost of catering to ill-founded complaints. If the problem is
fundamental limitation in technology rather than design, it would be a
waste of everyone's time (mines and theirs) to continually try different
things (it takes a chunk of time to drive out there).

I printed out Ian's details about fit of lens inside frame, and physical
deformation of the lens by the frame. She's pretty adamant that they
avoid that.

I also showed her Robert's ideas on proper fit, and "abbe". She politely
assured me that those are the things that she is trained in. In fact, the
move from n=1.67 to 1.6 already implements one of the suggestions, though
I'm sure it's not Seiko's n=1.6 aspheric lens -- this 2nd attempt at making
the lens included moving to a spherical lens to replicate conditions of my
old glasses as much as possible. However, since 3 parameters were changed
at once, I'm not entirely sure which actually contribute to the
improvement, and which may limit the improvement. I suspect that the
higher base curve helps, for the reasons above (sure sounds logical). As
well, I suspect that the lower index helps, since this seems to be the
common opinion among those who work in this area. However, I wonder about
the move to a spherical lens. Isn't the aspherical design suppose to make
the clarity of sight more uniform throughout the entire field of view, when
looking straight ahead as well as looking off-axis?

I also looked up abbe. It seems to be another way to characterize the
variation of refractive index with color, so now I see why Robert mentioned
this in the context of "transverse chromatic aberration". I should have
asked if it was a low-abbe material. Perhaps I'll call back to ask.

Thanks for your suggestions and information. If you have any further
comments on the thoughts I've written here, I would certainly welcome them.

Fred

 

Oblique astigmatism can be minimized by using the correct "best form"
base curve and by positioning the lenses properly in front of the
eyes. For any given Rx and lens design there is one correct base
curve. The reason for the off-axis blur is a combination of the low
abbe value of 1.67 index and/or a poor fit.

If you use the wrong base curve there will be an increase in oblique
astigmatism and spherical power error. If these lenses were in
"finished form", meaning off the shelve and ready to be cut and edged
into the frame, the base curve will be correct. If the lens was
surfaced or ground by the lab then there is a chance the wrong curve
was used. The center thickness is about 1.0mm with finished lenses and
usually no less than 1.3mm if surfaced, and can be (and usually are)
much thicker than that depending on good the lab is. Have the optician
check the center thickness also.

If you use a different base curve than called for by the manufacturer
of the lens then there will be an increase in off-axis blur.

Aspheric designs allows the use of flatter base curves without
increasing oblique astigmatism. Flatter front curves reduce the weight
and thickness of the lenses. No reason not to use an aspheric design
just because your old lenses were not aspheric, unless the optician is
inexperienced. Aspheric designs require more accuracy when positioned
in front of the eyes. The off-axis blur is due to the low abbe and/or
poor frame/lens fit.

As I mentioned in my original post, the abbe value of 1.60 index
varies from 36 to 42. The newer and better lenses are 42.

Hope this helps






Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical

~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman

 

Yes, it does. The lab guy also phoned me and corrected
the terminology I should google for. It should be
"oblique incidence astigmatism", not "oblique[ly] induced
astigmatism". That turns up some useful information.
Thanks for your additional information about the sensitivity
to positioning, abbe & tranverse chromatic aberration,
sensitivity to base curve, and background on pre-prepared
lenses. I have to figure out what I am going to do. I
haven't been able to wear these lenses for extended
periods (can be disorienting). As well, the left lens
is heavier with the spherical lower index material, and
tends to sag. That doesn't help it's positioning, I
suspect. Even properly positioned, I'm finding that
the left vision on-axis is blurrier than the right eye.
Not by much, but noticable. I'm hesitant to bug the
optmetrist about checking the accuracy of the my eyes.
I know that the lens matches the prescription accurately,
because I had it checked.

Thanks again for your additional details.

Fred