Contact lens prescriptions- what do the numbers means

[Elizabeth Mair]

Could an optometrist please explain what the numbers on contact lens prescriptions means. I understand the power numbers but not the rest.

eg -3.50/-0.75 x 40

I'd also like to know what removing the cylinder from a prescription means.

[Lynn White]

-3.50/-0.75 x 40

OKay

This means the eye has 3.50 dioptres of myopia (shortsightedness) and 0.75 dioptres of (myopic) astigmatism at an axis of 40 degrees.

Astigmatism means that the eye has more power in one meridian than the other at 90 degrees to it. So this prescription means that the eye has -3.50 dioptres in one meridan and -4.25 in the other.

Does this help?

Lynn

[Elizabeth Mair]

Thanks Lynn,
Does removing the cylinder mean taking out the allowance for astigmatism to make a basic lens, so that the prescription can be sorted out? There wasn't a fitting set available the day that I was at the clinic.

Why are there 2 different ways of writing prescriptions for glasses? It causes confusion.

[Lynn White]

Yes you are right Elizabeth - fitting a spherical lens will establish fit and so on - the astigmatism can be corrected over the top with trial lenses to see how your vision comes up.

By "two ways of writing prescriptions" do you mean sphere and cyl?? Because we HAVE to have those two things to actually make glasses etc - can't be done without it!