Understanding Corrective Lenses: Examining Reversed Prescriptions

[Flyboy]

My girlfriend has started wearing corrective lenses to address some nearsightedness and I'm not fully understanding what she's going through, or what the difference is for her with and without glasses. I was curious if you basically reversed the prescription of the lenses, would it take my normal vision and give me an idea of what she's seeing without the lenses?

 

[JT Smith]

I believe that to first approximation that the diopter powers of lenses are additive. So if your friend has a correction of, say, -3.0 diopters you should be able to get a sense for what her uncorrected eyesight is like by wearing +3.0 "reading" glasses. You can find those at a pharmacy. They don't require a prescription and they are inexpensive.

Or maybe I'm wrong.

 

[jrmichler]

Or maybe I'm wrong.

Nope, you had it right - diopters add. However, there are complicating factors. She may have some astigmatism. Her eyes might have different range of focus from yours, so your eyes might partially compensate for the reading glasses.

A better test is to compare the distances at which you and she can read, for example, street signs.

 

[berkeman]

You can find those at a pharmacy.

And they are typically just sitting in open display on a shelf or at a small kiosk. They are meant to be tried on, so you don't even have to buy a pair to get an idea. Just look at your GF's Rx and as mentioned above, pick the pair or reading glasses with the complementary Rx in diopters and look around as you stand there. Can you say what her optical Rx is?

 

[Flyboy]

Not off the top of my head, wanted to make sure I was understanding the physics right.

I do know she has a little prism to hers, which I don't think reading glasses have, but the reading glasses is a good idea for an approximation.

 

[Nik_2213]

If 'prism', that suggests a degree of astigmatism. Try rotating her glasses while looking through them. The result may be disconcerting as eg verticals blur vs horizontals, then vice-versa...