dive prescription

[melcas]

My friends I want take diving classes but I don't know if I have to adjust my prescription for underwater or is the same that I need to use.

-4.75-1.75 x 180
-5.25-1.50 x 007

My actual prescription is higher and I need to see the beautiful creations.

[Alvaro Cordova]

It all depends if you are using an insert like a scuba spec for example. If not, you will have to change it [the Rx] and I'm sure Darryl will enlighten. :)

edit: spelling, grammer and brackets

[chip anderson]

I have little expertise in this but the indice of water (and salt water again) is different than air and the Rx need be adjusted. However, in water one probably need see no more than 30 feet.


Chip

[Darryl Meister]

If you are using inserts, it won't matter since the lenses are not exposed to water. Otherwise, you could have your prescription made with a flat (or "plano") front curve. This wouldn't be entirely out of the question with your prescription, and you could then use the same lenses in both air and water. However, this approach would limit your field of perfectly clear vision slightly. The other approach would be to have the front surface power of the lenses "compensated" for the optical effect of water (this would require you to see an eyecare professional who is both willing and able to do this for you). This approach would offer a slightly wider field of clear vision under water, but would result in blurred vision out of water.

[Alvaro Cordova]

I think if we used an example that would clear things up. It would for me. I remember compensating for the front curve and got something ridiculous. Let's take Melcas' Rx as an example

-4.75-1.75 x 180
-5.25-1.50 x 007

on say a 2 BC OU in poly

This is what I get for compensating the front curve:

0.293 = (1.586 - 1) / 2.0

0.87 = (1.586 - 1.33) / 0.293

Is this right?

[Darryl Meister]

Yes, that gets you off to a very good start. You generally have to choose from a range of fixed front (or "base") curve values. Consequently, you would most likely start with a known base curve and then change the required back curve based upon the new effective power of the front curve in water.

So, given a prescription like -4.75 -1.75 x 180 on a polycarbonate 2.00 base, the effective power of the front curve would be:

Effective Front = (Poly Index - Water) / (Poly Index - 1) * Original Front

Effective Front = (1.586 - 1.333) / (1.586 - 1) * 2.00

Effective Front = 0.86 D

And, the approximate (or "thin lens") power of the back curve would be:

Back Curve = Prescription - Front Curve

Back Curve = -4.75 - 0.86

Back Curve = -5.61 D

For simplicity, I have assumed that the base curve is quoted in the index of polycarbonate. However, in reality base curves are quoted in a 1.530 index, which would require an additional conversion. Also note that this lens would be off by a considerable amount when worn in air, which means that the wearer's vision would become extremely blurred once she hit the surface in these goggles.

[chip anderson]

When they hit the air they only have to be able to find the boat. Acuity need not be too keen. Available light after you get down about 30 feet, makes vision beyond 30 feet not too necessary.

[Alvaro Cordova]

Ahhhhh. OK I see you used the radius of curvature formula like you showed me in the other post.

R = (n - n') / D

(n1 - n1') / D1 = (n2 - n2') / D2

D2 = ((n2 - n2') / (n1 - n1')) * D1

n2 = n1 = 1.586

And of course one would further compensate for the lens clock @ 1.530 after everything is done.

Wow thanks, You have a great talent for explaining things.

[Alvaro Cordova]

When they hit the air they only have to be able to find the boat. Acuity need not be too keen. Available light after you get down about 30 feet, makes vision beyond 30 feet not too necessary.

Very true, but this is a great exercise in optics. :bbg:

[Darryl Meister]

Wow thanks, You have a great talent for explaining things

Thanks, Al.

When they hit the air they only have to be able to find the boat.

And they won't be able to do much else. In our 2.00 Base Poly example, the wearer's prescription would be off by about 1.14 D in air (and even more for steeper base curves). This may make it more difficult for her to see smaller objects floating on the surface (such as a line), hand signals from anyone on the boat, and that sort of thing. And keep in mind that, with this high of an Rx, this wearer would also have to keep her goggles/mask on in order to see until she reached her dress eyewear.

I would probably suggest that the use of the flattest base curves feasible in this case (moderate to high minus); the flatter the better, since the error from the desired prescription in air will be roughly equal to half of the base curve.

[chip anderson]

Darrel: One can see quite well with only 1 diopter less minus than required.

Most folks with -1.00 Rx can see 20/30 or so with no correction.

[Snitgirl]

Give a call to this company. I am not affiliated with them but know of their work. The owners name is Linda. www.prescriptiondivemasks.com

My friends I want take diving classes but I don't know if I have to adjust my prescription for underwater or is the same that I need to use.

-4.75-1.75 x 180
-5.25-1.50 x 007

My actual prescription is higher and I need to see the beautiful creations.

[Darryl Meister]

Most folks with -1.00 Rx can see 20/30 or so with no correction

I would put 1.16 D of power error at closer to 20/60 under "typical" conditions for the "typical" observer, though an individual might get to 20/40 or better on an especially sunny day (since a reduction in pupil size will improve vision some). I wouldn't call 20/40 vision "seeing quite well" though, and this represents the limit of uncorrected vision for driving in most states. While 20/40 may allow for functional vision in most cases, I suspect that our diver will still have a harder time finding that rope on the surface of the water.

[CME4SPECS]

I see no mention in the above posts regarding vertex distance. The change of vertex distance is probably going to have more effect on this rx than calculating the rx for water or air. You will need to add more minus (probabally in the 1 diopter range) to compensate for the change in vertex distance.

[Jacqui]

You might also check with Sport Optix http://www.sportoptix.com/ they sell presription diving masks. Tell them CD Optical sent you.

[Alvaro Cordova]

I see no mention in the above posts regarding vertex distance. The change of vertex distance is probably going to have more effect on this rx than calculating the rx for water or air. You will need to add more minus (probabally in the 1 diopter range) to compensate for the change in vertex distance.

for 5mm going out the Rx isn't affected that much
original
-4.75-1.75 x 180
-5.25-1.50 x 007

5mm out
-4.8655567 -1.8527894 180
-5.3915277 -1.5942421 007

10mm out
-4.9868765 -1.9649949 180
-5.540897 -1.6977091 007

20mm out
-5.2486186 -2.2226453 180
-5.8659215 -1.9375467 007

[CME4SPECS]

I have bonded many glass lenses to masks. Most vertex distances need to be compensated in the 10-20 mm range.

[Darryl Meister]

Most vertex distances need to be compensated in the 10-20 mm range

In any case, it is certainly an important consideration.

[Ed_Optician]

High volume masks....the old Jaques Cousteau single plate mask has a longer vertex distance than more modern Low volume masks. Regardless of the mask today I would still surface on a plano base crown glass lens and use UV curing optical cement to laminate it to the faceplate. Measuring vertex will be a bear though assume a normal refraction at 12mm mask fits at 20 in this Rx it is well under.25 D error. Dont try to compensate for water even emmetropes have the effect of everything feeling 25% closer than above water

Ed