Growing up in Asia, optometrists always used a system of “200 degrees, 400 degrees, 800 degrees” for nearsightedness. In America, however, it’s apparently “diopters” of “-2.5, -4.0, -7.5,” etc.
I’m not sure how it converts. Does “-2.0” diopters in the USA mean Asia’s “200 degrees of nearsightedness?”
I’m purchasing nearsighted goggles for swimming, but they only come in equal-diopter calibration for both goggle lenses. My left eye is 400, my right eye is 200 according to the Asian system. Would it be better for me to buy -2.0 diopter goggles to accommodate my better eye, -3.0 as some middle ground, or -4.0 to accommodate my worse eye?
I’m afraid I’m unfamiliar with the use of degrees to characterize myopia. I’ll look into it.
In the US and Europe, the prescription you’re given tells the optical power of the glasses needed to correct the vision. “Power” is the inverse of the “focal length” of the correcting lens, expressed in meters. A 1 Diopter lens ( 1 D) has a focal length of 1 meter. A 2 Diopter lens has a focal length of half a meter, or 500 mm. A 0.5 D lens has a focal length of 2 meters.
The reason that things are expressed in this weir way is that optical powers add. If you put a 0.5 D lens next to a 1 D lens, you get a 1.5 D lens, which has a focal length of 2/3 meters.
Putting a negative sign in front means that it’;s a negative lens (which is what you need to correct myopia). So a -2D lens has a focal length of -(1/2) meter, or -500 mm.
As for your swim goggles, you can get something that works for your eye that needs less correction and leave your other eye a bit blurry. I’m not sure having both eyes equally afflicted is a good solution.
You can always glue in an extra lens to give more optical power to the more afflicted eye.
This page seems to be saying that 200 degrees of myopia corresponds to 2 Diopters, and that, in general N00 degrees is N diopters. But correcting myopia requires a negative lens, so your lens should be -2D to correct 200 degrees of myopia.
So if you have one eye 200 degrees and the other 400 degrees, one possibility is to get swim goggles with -@ Diopters of correction and get another -2 D lens from somewhere (Edmund optics, maybe) and attach it. Don’t glue it by the center, or you’ll screw up the optical correction – glue it at the edges.
For myself, I was going to get a pair of normal swim goggles and just glue in corrective lenses, until I found out that they made corrective ones. I then bought a pair that worked for one eye and didn’t worry too much about the other. My eyes are different, and both are astigmatic, so the single-power lenses didn’t even properly fix one eye. But it never made any difference that I could see.
I too would recommend correcting for the better eye and letting the other one be a bit blurry. Your brain will sort it out. Overcorrecting your better eye to get the worse eye in focus seems like a recipe for headaches to me.
When I had my goggles made, I needed 9 diopters of correction (I’ve since had LASIK) but the manufacturer only went up to 8. They worked great because the water itself provides some degree of correction.
Snorkel-mart lets you choose different prescriptions for the left and right eye.
I have slight astigmatism and need bifocals, but I just had mine set to my distant vision and they work good enough.
Another idea to consider is to buy one pair of each prescription and swap out one gogglette (?) on a pair for the other. Sure, twice as much money but a better solution overall.
All the swim goggles I’ve used in recent years have readily removed … gogglies (?).
The strength of the lenses that you get is different in swim goggles than it is for air lenses (as @Nars_Glinley mentioned, the water changes the correction). Ask your optometrist to give you a prescription for swim goggles.
I don’t think that this is correct. People use the swim goggles for seeing in the air while they are swimming (the goggles are to keep the water out of your eyes). I don’t know of anyone who makes corrective lenses intended for seeing underwater, as in a diving mask.
I’ve seen papers about such lens/diving mask combinations being made. They look completely different from lenses that would be used above the water, because the refractive index of the water (1.33) is so large compared to that of air (1.0).
Your eyeballs are in the air zone while in goggles/mask, but you’re focusing on objects several feet away through water.
I just got back from Hawaii in May, where I brought goggles and diving mask which had been ordered based on my snorkeling optometrist’s recommendation. See here:
We’re talking about two different things. I understood the OP’s quuestion to be about the goggles used by surface swimmers to keep their eyes clear of water. As my last post makes clear, diving goggles are a different matter.
It’s pretty clear that your link is also about swimming goggles used by surface swimmers, not snorkelers.
If you want goggles yo use underwater, you really do want something very different, and which I don’t think is commercially manufactured. Your best bet for a quick solution is to wear your regular glasses inside a diving mask big enough to accommodate them.
I don’t know what divers with severe myopia or presbyopia do. I’ll have to look into it.
That well may be - I just grabbed the first link on my phone I saw that looked likely. I obviously have only a very superficial understanding of the concepts.
I do have severe presbyopia and astigmatism, and got underwater goggles and mask that worked quite, at a different strength than I normally wear. I don’t believe it’s possible to wear eyeglasses inside a normal diving mask.
As long as the outer surface is pretty flat, the same corrective goggles ought to work eithrer in air or in water – all the optical power comes from the inner surface, and the outer, flat surface has no optical power.
Of course, virtually all swim goggle have a somewhat curved outer surface, but it ought not to be a big contributor.
My statemrents in my first post still stand, though.
One thing that may come into play and be different between eyeglasses, contact lenses, swim goggles, and a dive mask: vertex distance:
The stronger the lens – generally, in plus or minus power (ie, sphere) – the greater the impact of vertex distance on the image perceived by the wearer.