Understanding eyeglass prescriptions
Bill Kinnersley
ophthamologist, and received from him an eyeglass prescription that
seems to be quite different from her previous one. However, if my
interpretation of the numbers is correct, it is not so different
after all. Could someone please either confirm or deny this for me?
Old prescription:
Spherical Cylindrical Axis
O.D. -2.75 +1.00 25
O.S. +0.50 +0.75 135
New prescription:
O.D. -1.75 -0.75 120
O.S. +1.25 -0.50 60
What I think this means, for the right eye for example, is that the
old curvature is -2.75 along the 25 degree axis and -2.75 + 1.00 = -1.75
along the 115 degree axis (90 degrees away), whereas the new curvatures
are -1.75 along the 120 degree axis and -1.75 + -0.75 = -2.50 along the
30 degree axis.
--
--Bill Kinnersley
bi...@eecs.ukans.edu
226 Transfer complete.
JIM8749
> However, if my
>interpretation of the numbers is correct, it is not so different
>after all. Could someone please either confirm or deny this for me?
>Old prescription:
Spherical Cylindrical Axis
>O.D. -2.75 +1.00 25
>O.S. +0.50 +0.75 135
>New prescription:
>O.D. -1.75 -0.75 120
>O.S. +1.25 -0.50 60
Transposing the old Rx you would get
O.D. -1.75 -1.00 x115
O.S. +1.25 -0.50 x045
You are correct in stating that there is very little change.
Jim
Steve Machol
: Old prescription:
: Spherical Cylindrical Axis
: O.D. -2.75 +1.00 25
: O.S. +0.50 +0.75 135
: New prescription:
: O.D. -1.75 -0.75 120
: O.S. +1.25 -0.50 60
: What I think this means, for the right eye for example, is that the
: old curvature is -2.75 along the 25 degree axis and -2.75 + 1.00 = -1.75
: along the 115 degree axis (90 degrees away), whereas the new curvatures
: are -1.75 along the 120 degree axis and -1.75 + -0.75 = -2.50 along the
: 30 degree axis.
Exactly right, Bill. The confusion comes because some Doctors prefer to
write prescriptions in "plus cylinder" form (as in the Old prescription)
while some prefer to write them in "minus cylinder" form (i.e., the New
prescription.) You can transppose the Rxs from one cylinder form to
another by doing exactly what you've done here.
Regards,
Steve Machol
Sola Optical USA
James Hughes
>Old prescription:
> Spherical Cylindrical Axis
>O.D. -2.75 +1.00 25
>O.S. +0.50 +0.75 135
>
>New prescription:
>O.D. -1.75 -0.75 120
>O.S. +1.25 -0.50 60
If we change the old Rx to the same format of the new Rx they are quite
similar:
Old Prescription converted:
OD -1.75 -1.00 x 115
OS +1.25 -0.75 x 045
By comparing numbers it appears the two prescriptions are very similar.
Dennis Yelle
Is it a fact of optics that two cylindrical lenses with axis 90 degrees
apart are equivelent to a single spherical lens? Or is this just an
approximation? If it is just an approximation, how far off is it?
It is really difficult for me to imagine how they could be exactly equivelent.
--
den...@netcom.com (Dennis Yelle)
"It's a small mind that can think of only one way to spell a word." -- M. Twain
BillyFish
*********
My wife has astigmatism. She recently started going to a different
ophthamologist, and received from him an eyeglass prescription that
seems to be quite different from her previous one. ...
*********
The problem with eyeglass prescriptions is that more than one
specification can be used to devine the same lens. For example, two
positive cylindrical lenses at right angles to one another is the
equivalent of a spherical lens.
The reason for this, in mathematical lingo, is that unormalized
non-orthogonal functions are used to describe the lens.
There is a set of orthonormal functions for describing lenses, the
Zernicke polynomials. The amplitudes of the first few terms would
describe conventional lenses uniquely. The principal terms would be
represented by
x-wedge not usually prescribed
y-wedge not usually prescribed
sphere
x-hyperbola
45 degree hyperbola
It is a simple matter to convert an ordinary prescription into the three
components uniquely, but the reverse is not true. Moreover, the
hyperbolic correction does not exactly correspond to cylindrical
correction. Although I do not grind and polish lenses, I do not know if
it is harder to produce hyperbolic correction than cylindrical correction.
My guess is that the profession does not wish to change a system that has
worked for a long time into one that has more mathematical elegance to it.
William Buchman
James Hughes
-1.00 -0.75 x 090 is equal exactly to:
-1.75 +0.75 x 180
dick.cl...@fovea.com
>ophthamologist, and received from him an eyeglass prescription that
>interpretation of the numbers is correct, it is not so different
>after all. Could someone please either confirm or deny this for me?
Are you studying to be an Optician, Optometrist, or Ophthalmologist? I'm
all for an informed and educated public, but patients who attempt to second
guess what the doctor prescribes are asking for trouble. It seems that
there is a segment of the population who, because of negative articles and
lack of trust, now look up every prescription in the PDR, or compare every
finding from previous exams. I've had patients return with advanced
glaucoma who respond, "I didn't want to take the drops after I read about
them in the Physician's Desk Reference," or I didn't fill the last glasses
prescription because it didn't look right. I also cringe when a patient
tells me their exact prescription as if they had a concept of what the
numbers meant, and then describe their astigmatism as if it were an eye
disease.
If you're concerned about a change, ask the doctor who prescribed the
glasses. If you don't trust him, or you don't feel confident in what he's
done, then get a second opinion. But, remember that a layperson who treats
himself has a fool for a doctor. Doctor's are not perfect, and they can
make mistakes, but if you second guess every exam, then it's time to find
another doctor. Again, don't get me wrong; I think patient education is a
very important part of the treatment process. I just think it's a bit short
of teaching a course in optics or pharmacology.
Dennis Yelle
>Again, don't get me wrong; I think patient education is a
>very important part of the treatment process. I just think it's a bit short
>of teaching a course in optics or pharmacology.
Well, for a start, I wish the doctor would tell me things like,
"we will check the right eye now."
"now we will check the left eye."
"now we will check for astigmatism in the left eye."
"now we will check for astigmatism in the right eye."
If the doctors would get in the habit of doing this, I would
be able to talk about the exam at the end.
As it is usually, I have no unabiguous way of telling the doctor
when something happened. The best I ever came up with was,
"I liked the lenses that I was looking thru the first time I read
most of the bottom line of your chart. I wish I could go back
and get those, instead of these. Actually, I wish that you had
stopped the exam at that point, because I think that my eyes are
'screwed up' some how after looking thru all of those distorted
lenses." (The bottom line was the 20/20 line, but I didn't know
that at the time.)
Charles Lindquist
>
>The problem with eyeglass prescriptions is that more than one
>specification can be used to devine the same lens. For example, two
>positive cylindrical lenses at right angles to one another is the
>equivalent of a spherical lens.
>
In a few words or less, what is a "slab-off" lens? This was prescribed
for myself for one bifocal.
Just curious! Thanx in advance.
charles lindquist.
--
Charles Lindquist Grass Vally Group, Inc X3729 (478-3729)
char...@aurora.gvg.tek.com char...@treehouse.org
"An Engineer should be a cultured mind, not just a handy tool"
Nathan F. Dupuis
Steve Machol
: In a few words or less, what is a "slab-off" lens? This was prescribed
: for myself for one bifocal.
A slab-off lens is sometimes prescribed when there is a relatively large
power difference between the Rxs of each eye. In a "slab-off", one of the
lenses is cast or ground with prism in the near portion of the lens,
leaving the distance portion unchanged. This prism is designed to correct
the vertical imbalance through the reading area of the lens. Without the
prism, the eyes would try to converge at different spots causing
noticeable discomfort while reading.
Regards,
Steve Machol
Manager, R&D Lab Operations
Sola Optical USA
JP Ward
You asked for simple terms. Imagine looking through two lenses (one for
each eye.) One lens is for farsightedness, thick center, thin edges. The
other is for nearsightedness, thin center, thick edges. Through the center
you look at a dot on the wall. Since you are looking through the center of
each the image is not moved from the center of your field of view BUT as
you move your head and keep your eyes fixed on the dot you start to notice
that the single dot becomes two. This is because the image of the dot is
displaced to different points in the eye since the lenses cause an
imbalanced displacement due to unlike powers.
In a bifocal, if you have unlike powers, whether far or nearsighted, and
you look through the center of the lens, you see a single image BUT when
you go down to the bifocal, you get this same effect described above. The
correction is a SlabOff. Quite literally, a lens with a slab ground away
from one of the lenses in the bifocal area, creating a prism that
displaces the image to the correct position so that you now see your near
image binocularly. This lens is also called a bi-centric lens.
Jack Ward
optician
Ft Lauderdale