parsing a glasses prescription

[cellio]

Previous prescription:
OD: sphere +8.75, cylinder -2.00, axis 135, add power +3.75
OS: sphere +6.50, cylinder -2.50, axis 45, add power +3.75

Newly-generated prescription (this morning):
OD: sphere +8.50, cylinder -2.00, axis 135, add power +4.00
OS: sphere +6.50, cylinder -2.25, axis 45, add power +4.00

Setting aside the bifocal strength as just a cost of getting older, I note two differences: OD sphere and OS cylinder are both lower than they were. I didn't have the previous prescription with me and didn't think to ask my ophthamologist what these differences mean. Some of you told me last time that sphere is about myopia and cylinder is about astigmatism. (As is axis, which didn't change.)

My right eye is weak enough that measuring anything on it is kind of a crap shoot, so I note but am not especially concerned by numbers suggesting that it was measured to be a little less myopic than it was last time. But what is it likely to mean that, according to these numbers, my left eye has gotten a little less astigmatic? Or are we in "noise" territory?

Comments

[metahacker.livejournal.com]

Prescriptions are kinda hard to measure accurately. I don't think a quarter diopter is really significant...it's the limit of resolution of their machine, so if you 'really' were (e.g.) +3.85 diopters it might swing one way then the other.

Also, I've had bad experiences with getting prescriptions done where my eyes refocus to adjust to whatever prescription they are giving; then the prescription changes, but my eyes haven't *really*--instead I just end up with glasses that give me a headache as my eyes spend all day pulling focus.

[eub.livejournal.com]

Yeah, I don't know, but until I see a 0.125 I'm going to assume 0.25 is their precision limit.