eyeglass prescription question

Hi,

I have a question about my eyeglass prescription. I am near-sighted and
have some amount of astigmatism in both of my eyes.

My question is about the prescription. It is written as

Sphere Cylinder Axis
OD -7.25 +0.50 88
OS -7.25 +1.50 98.

I am confused about the Cylinder values. Isn't it true that
near-sighted people like me should have negative cylinder values
instead of positive ones? Please let me know what the cylinder values
mean. Thanks!

Best Regards,
Doug

 

Optometrists usually write in minus cylinder form versus
the plus cylinder form (as in your rx) written mostly
by opthalmologists. Basically, you can transpose (switch)
into either form using a simple formula. The cylinder
values just express the amount and orientation of your
astigmatism (irreg curve of your corneas).

frank

 

You may translate your prescription as follows, it is still the same power.
Notice the - (minus) sign and the change in degrees (90degree difference)
All to international agreements (TABO)

OD S-6.75 = C-0.50 178
OS S-5.75 = C-1.50 8

In Europe we are used to prescribe with the - (minus) sign notation for the
astigmatic part of the prescription.

 

I and others here have written here about this subject before. With
computers being used more and more, the confusion is readily soluble. A
computer can be use to translate from sphere/cylinder/angle to sphere/0°
astimatism/45° astigmatism or back. The first form is ambiguous. The second
form is not. The first form can become the legacy form used to interpret old
prescriptions. Both forms can be put onto a computer written prescription.

The second form uses two orthonormal Zernike functions to represent
astigmatism. Using that allows only one unambiguous value for the spherical
correction. These Zernike functions can only be approximated using
cylindrical lenses. These days however, with wavefront correction in vogue,
even spectacle lenses may end up being corrected for higher order
aberration.

This gets me to a question about the preparation of spectacle lenses. Are
such lenses fabricated the same way irrespective of wether
optometrist/ophthalmologist notation is used?

Bill

 

Yes.

 

So how is it done? Is only one surface shaped? Is the sphere on one side of
the blank and cylinder on the other? If this last case is true, how does the
optician set the sphere?

Bill