Should corrective power of eyeglasses and contacts be different?

Question

My optometrists' office is closed today, so I'm asking here, my eyeglasses prescription specifies a "spherical" of -4.75, and my contact lens prescription specifies a "power" of -4.50. Both prescriptions were from the same examination. Before I order contacts, I want to know if this discrepancy is normal, or a clerical error?

Answer 1

No, that would be quite usual, at that power.

The glasses sit at some distance from the cornea, and that "vertex distance" changes the effective power at the eye and needs to be factored in.

Under about +/- 3.00 it's usually too small to bother about, but at higher powers it can get considerably more than 0.25.

A +10.00 spectacle lens 12mm from the eye equates to a +11.50 contact lens on the eye...  See chart at bottom of URL below.

+ve, contact power higher, -ve spectacle power higher.

With rigid contacts, there are more issues that can lead to a greater difference.

Answer 2

I don't think so, my perscripiton for both is 4.50.

There are variables though, like if you have a astigmatism changing the shape of your pupil, but if you ever wear contacts, you'd know that by now.

Be patient, don't order your contacts till you talk to your doc!

Answer 3

That is correct like that.

a -4.75 in glasses has to be reduced by 0.25 for contacts...that's done to give you the same power because of the vertex distance from your eye.

Conversion like that starts when the glasses Rx reaches -/+ 4.50

Answer 4

It is not that unusual for the contact perscription and glasses perscription to be slightly different, especially in the higher strengths. The lenses of contacts sit right on your eye, whereas the lenses of eyeglasses sit spaced away from the eye, this has a slight effect on the optics.

If you can survive wearing glasses another day or 2, you could double-check with their office to be sure.