# Help me understand my eyeglasses prescription

**URL:** <https://boards.straightdope.com/t/help-me-understand-my-eyeglasses-prescription/801063>\
**Category:** Factual Questions\
**Created:** [November 8, 2017, 5:13pm UTC](https://boards.straightdope.com/t/help-me-understand-my-eyeglasses-prescription/801063 "2017-11-08T17:13:46Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![gaffa](https://avatars.discourse-cdn.com/v4/letter/g/e68b1a/32.png) [@gaffa](https://boards.straightdope.com/u/gaffa)\
**Post date:** [November 8, 2017, 5:13pm UTC](https://boards.straightdope.com/t/help-me-understand-my-eyeglasses-prescription/801063/1 "2017-11-08T17:13:46Z")

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I have a prescription for my glasses. I should be getting bifocals, but I can’t afford them. The last time, I managed to get two different types - one with my distance prescription and one for close-up. I lost my close-up glasses, and I don’t know how to order a replacement pair from Zenni Optical. Here’s my prescription:

```auto

                     (SPH) (CYL) Axis NV-ADD
OD - Right -3.50 -1.00 60
                                                                         +2.00
OS - Left -3.25 -1.00 75
           PD 69

```

Do I subtract the NV-ADD number from the SPH value? That would make the right eye SPH -1.50 and the left -1.25?

Yes, I know I should buy from my eye professional, but I just don’t have the money. Please help.

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**Author:** ![Inner\_Stickler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inner_stickler/32/318_2.png) [@Inner\_Stickler](https://boards.straightdope.com/u/Inner_Stickler)\
**Post date:** [November 8, 2017, 5:20pm UTC](https://boards.straightdope.com/t/help-me-understand-my-eyeglasses-prescription/801063/2 "2017-11-08T17:20:52Z")

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> [@](#):
>
> It is easy to reconfigure an Rx with an NV-ADD. It just involves a little grade-school arithmetic.
> 
> You simply take the NV-ADD number on your Rx and add it to the numbers in your Sphere (SPH) category for each eye. Then lower your distance PD by 3 millimeters if it’s the single PD, by 1.5 millimeters for each eye if it’s the dual PD. That gives you an Rx for single-vision reading glasses!

[http://www.zennioptical.com/faq/prescriptions?count=5#faqTopic](http://www.zennioptical.com/faq/prescriptions?count=5#faqTopic)

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**Author:** ![gaffa](https://avatars.discourse-cdn.com/v4/letter/g/e68b1a/32.png) [@gaffa](https://boards.straightdope.com/u/gaffa)\
**Post date:** [November 9, 2017, 12:28am UTC](https://boards.straightdope.com/t/help-me-understand-my-eyeglasses-prescription/801063/3 "2017-11-09T00:28:25Z")

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Cool, thanks.