I'm farsighted, my lenses magnify....and I can use dollar store glasses as prescription glasses with no problem. My eyes have stabilized at + 2.75, but 30 years ago I was +1.0, then +1.25, then.....a gradual progression.
I tried using corrective lenses made of plastic inside the mask. They correct just fine but they fog and I could not find a way of preventing the fogging.
So, I tried putting the lenses on the outside of the mask...in the water. Back when I was +1.00 that method worked just fine. But the lenses needed to have more correction than if used in air. Now that I'm +2.75 it takes lenses whose diopters add up to 10.50 to see well in my hot tub, mounted on the outside of the mask.
Here's why. The Index of Refraction is 1.000xx for air, 1.56 for plastic lenses (unless using high-index plastic) and 1.33 for water. The thing that matters is the difference in Index of Refraction, IOR, between that of the plastic lens and that of the fluid it is imersed in. Plastic minus Air IOR is 1.56-1.00=0.56. Plastic minus water is 1.56-1.33=0.23. Plastic lenses imersed in air have an effective IOR difference of 0.56 but only have and effective IOR difference of 0.23 when imersed in water.
The ratio of effective IOR differences between air and water is IOR Air / IOR Water. 0.56 / 0.23 = 2.43. For a magnifying lens to do the same thing in water that it does in air it's diopter needs to be 2.43 times as much. For example, when I needed +1.00 diopters in air...I needed +2.50 diopters for the lenses mounted on the front of my mask, the lenses that were imersed in water. That is +1.00 diopeters x 2.43 = +2.43 diopeters. Actually I used +2.00's glued to the front of the mask, with space for the water to circulate between the glass faceplate of the mask and the back side of the glued on lens.
Now it is different. I need +2.75 diopters in air and +2.75 diopters x 2.43 = +6.68 diopters imersed in water and attached to the front of my mask.
(Note: you are always better off being a bit under corrected than overcorrected...ie. better off being a bit farsighted.)
Hmmm, how to get +6.50 diopters? Turns out it is easy. You can stack up magnifying lenses and the resulting diopeters are roughly the add-em-up SUM of the diopeters of the individual lenses. So, take two dollar store +3.25 diopter glasses, attach em together after first cutting off the ear pieces and then attach em to the front of the mask...viola a mask with good correction. The lenses can even be mounted backwards...since they work OK looking through from either the front or back side. Better, it only costs $2 to try it. Need a different correction, fine just use different combinations. For close up you can use a lens stack that gives a higher diopter sum.
On the other hand you can buy a mask with out of the box correction for about $63.50 delivered from Snorkel Mart, search the web. Scuba Toys has out of the box perscription masks as does Joe Diver and some other places. I get a lot of funny looks wearing a mask that has lenses glued to the outside. And the silicon glue does not always hold and the lenses are impossible to clean behind.
Contact lenses work just fine in diving masks, so that is an option. I use BL Pure Vision silicone lenses and can leave them in for days, even in my sensitive eyes. (Hint, just put the overnight solution directly in the eye to clear the lenses up.)
UW optics is interesting, huh?
