# Optical Forums > Ophthalmic Optics >  Rx swim goggles & astigmatism

## mlm

I had a discussion with our optician recently about prescription swim goggles.  He says that regardless of the script, all you need to do is order spherical equivalent powers because being underwater will cancel out the need for astigmatic correction.  I admit my knowledge of optics isn't vast, but I just can't figure that out.

I can see that maybe working for low astigmats (up to -0.75 or -1.00 cyl perhaps).  But since astigmatism is either corneal or lenticular, I don't see how being underwater would make any difference in correcting for it.  I really can't figure out how someone with say -3.00 -2.00 x180 would be able to see through a -4.00 lens.

This is all a theoretical discussion, but I want some input before I get back into the fray with him.  Can anyone clarify this for me?

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## HarryChiling

Simply put not true.  Although keep in mind that the index of water is higher than the index of air and if you are going to make the script according to the doctors Rx you have to account for the index changing on both sides of the lens.  This may be where he gets confused.

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## chip anderson

Rumor started with Leonardo DaVinci. He found that when he placed his face in a container of water and opened them his astigmatism was canceled. Still true but the the water must make contact with the cornea which it does not with goggles or mask on.
Same principal that rigid lenses (before they became so damn flexible) canceled most astigmatism with spherical base/powers.
However one should adjust powers of swim goggles for the index of water unless they are inserts with air on both sides inside the mask or goggles.

Chip

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## fjpod

One way to eliminate the interface between the plastic lens of a swim goggle and the water is to use a plano front lens.  This works ok with myopes and myopic astigmats.

Although being a swimmer and myope myself, I feel prescription swim goggles are almost useless.  What swimmers really care about is that they see clearly when they look out of the water after a swim.  The goggles have to come off because they are all wet. 

The real solution is contact lenses under the goggles.  You can see in and out of the water.

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## Ory

Keep in mind that you get a magnification effect from being underwater.  This may reduce how much you are bothered by your astigmatism.  A presbyopic hyperopic friend is happy with a non-prescription mask while diving because the gauges appear larger.

Why do you need crisp vision while in the water?  As long as you can see reasonably well, will you be happy?  If so, the best sphere may be acceptable.

I'm not trying to justify an obviously incorrect statement from your optician, but you may not need a custom solution if an off the shelf pair of goggles works.

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## mlm

Thanks everyone for your replies.  They confirmed what I suspected.

Ory, it's very true.  It would be rare that a swimmer would need crisp clear vision underwater.  And as fjpod pointed out, it's really vision when your head out of the water that matters.  When I posted the question, I was thinking about the original discussion in terms of the optics of cylinder correction, not practical real-world application.

I haven't had the opportunity to see a swim goggle dispense yet.  How do you go about adjusting the script to compensate?  (I guess if water magnifies, you add minus?)

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## Ory

> I haven't had the opportunity to see a swim goggle dispense yet. How do you go about adjusting the script to compensate? (I guess if water magnifies, you add minus?)


Magnification in and of itself would not really have an effect on the prescription.  From what I've seen (and as pointed out by fjpod), most companies that sell Rxable goggles/masks have pre-made plano-convex or plano-concave lenses in a variety of powers.  You can then simply pop in these lenses and voila! 

If you're desperate to glaze the goggles with the exact prescription including cyl I'd suggest looking for a plano front surface to save on the headaches.

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## Ory

deleted

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## HarryChiling

How many places in the world still have 20 feet of clear water to look through anyway?

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