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Prescription Glasses

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davewr | 10:36 Tue 26th May 2009 | How it Works
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I want to order reading glasses off the internet and when trying to put in the prescription details I've come across the following instruction: In the case of Positive Add and a Negative Sph, net the difference and the new number becomes the Sph.
Net the Difference is the bit I can't follow. The Sph is -0.5 and the Add is +1.75. Is the new number +1.25 or +2.25?
I've emailed the supplier but just got the same instruction sent back.
Thanks for any help.

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I suppose the net is +125

But why can't they just supply your prescription and give you glasses that suit your eyes? A +125 won't suit a prescription of -0.5/+1.75
This explains the terms a bit.
Is a prescription set very much cheaper online?
With something as vital as your eyes, wouldn't it make sense to go to a proper optometrist for the right lenses?
Question Author
I've had quite a few pairs off the internet with no problems and at a fraction of the high street prices.
In the past I've just given all the figures off the prescription and stated whether distance or reading and they've done the calculations.
The snag is that firms don't update the range of frames so you have to go to different firms if you want a change.
Thanks anyway.

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