In the past I’ve gone to an optometrist to get eyeglass prescription updates, who use negative notaton prescriptions. Recently I went to an opthamologist who uses positive notation. I want to make sure I understand the difference before I submit the prescription for buying glasses online.

My previous (negative) prescription was this:

OD (right) SPH -2.50, CYL -1.75, AXIS 5

OS (left) SPH -2.75 CYL -1.00, AXIS 175

Now I’m told my updated prescription is a minor change. It reads as follows:

OD Sph -3.75, Cyl +1.25 Axis 100

OS -3.75 Cyl +0.50 Axis 90

Now I see that the astigmatic cylinder has been rotated 90 degrees (actually 95 and 85) and the that the cylinder notation is now a positive number. Okay, so that’s part of the difference between positive and negative notation. There’s still a .75 gap between left ey and right eye, but both numbers are half a diopeter less in absolute value.

However, the spherical correction is now -3.75, still a negative number. Does this mean that the switch from negative to positive only affects astigmatism correction, or does the spherical number mean something else when listed in this method?

What confuses me is that they said my prescription was a minor change, yet the spherical number changed by a whole diopeter or 1.25 depending on eye, which does not seem like a minor change to me, which is making me think I don’t understand something.

Anyway, I order my glasses from zennioptical online. There’s no checkbox for giving your prescription in positive or negative notation. Does that mean I could just enter my prescription as-is, and they’d understand what was written and give me the right prescription, or do I have to do some sort of conversion to make it consistent with my previous negative-notation prescriptions?

Edit: I just realized that perhaps the increased spherical corresponds to the decreased absolute value of the cylinder corrections. So I get more spherical correction and less cylinder correction and maybe the numbers work out to be similar? Is that part of the difference between the notation, or is the doctor changing the relationship between my spherical and astigmatic correction?