Lost camera & strobes, end of Colombia
- Thread starter cicopo
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I researched this when I was looking to add a gauge/computer reading lens to my dive mask. I had to add it to the outside of my mask since the mask was already vision corrected with full sized lenses on the inside surfaces and the lenses I could get were flat on one side. A little drop of clear silicone aquarium sealer holds it on. I figured a +2 diopter would work, and on my bench it worked fine, but when I got in the water the added lens did practically nothing.
Come to find out, the difference in refractive index between a curved lens surface and the ambient fluid (air or water) determines how bent the light rays are when they pass through the interface, i.e., how much the lens magnifies the image. Air is the standard with a RI of 1.0. seawater's RI is about 1.35 and that of glass is about 1.5. Seawater and glass are so close to each other that the diopter of a glass lens in water is reduced by about a factor of 4. I needed a lens with a diopter of about +8 for it to work in water like a +2 lens in air. That's why corrective lenses are typically on the inside of a dive mask; the curved side of the lens needs to be in air for it to work like your eyeglasses Rx.
That concludes tonight's lecture on optics.
Come to find out, the difference in refractive index between a curved lens surface and the ambient fluid (air or water) determines how bent the light rays are when they pass through the interface, i.e., how much the lens magnifies the image. Air is the standard with a RI of 1.0. seawater's RI is about 1.35 and that of glass is about 1.5. Seawater and glass are so close to each other that the diopter of a glass lens in water is reduced by about a factor of 4. I needed a lens with a diopter of about +8 for it to work in water like a +2 lens in air. That's why corrective lenses are typically on the inside of a dive mask; the curved side of the lens needs to be in air for it to work like your eyeglasses Rx.
That concludes tonight's lecture on optics.
On line I found an equation or a chart (I don't remember which; it was a couple of years ago) that related diopter to either the difference or ratio (again, I do not remember which) between the refractive indices of media. It might have been simply a comparison of differences, i.e. glass minus air is 0.5 and glass minus water is 0.1-0.15 (depending on the type of glass and the salinity of the water), or three to four times less. Boost the diopter by that amount to compensate. Whatever is the math behind it, it worked.
Wow! The things I have learned on SB.
nwflyboy
Contributor
Picked up one of these when I had my prescription mask done:
Retractable Magnifier
Overpriced, but it does the job - it does make small things look bigger - and was easy to buy.
Retractable Magnifier
Overpriced, but it does the job - it does make small things look bigger - and was easy to buy.
Except for the retractor it looks just like the one I made myself for less than five bucks.Picked up one of these when I had my prescription mask done:
Retractable Magnifier
Overpriced, but it does the job - it does make small things look bigger - and was easy to buy.
AdivingBel
Diver
I've posted this before...My wife & I carry these... POCKET MAGNIFIER
I run a piece of monofilament through the little hinge and make a loop for clipping to a double ender. The cover protects from nicks & bangs (and makes a nice handle). I thought the little hinge would rust out after a year, but we've had the same ones for over 4 years. @Doc turned me on to them. Great conversation starter on a dive boat. Folks always ask me, "What's that tear drop thing clipped on your rig?" BTW, you really need one if you dive with Julio. 
I run a piece of monofilament through the little hinge and make a loop for clipping to a double ender. The cover protects from nicks & bangs (and makes a nice handle). I thought the little hinge would rust out after a year, but we've had the same ones for over 4 years. @Doc turned me on to them. Great conversation starter on a dive boat. Folks always ask me, "What's that tear drop thing clipped on your rig?"
BTW, you really need one if you dive with Julio.
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I've posted this before...My wife & I carry these... POCKET MAGNIFIER
I run a piece of monofilament through the little hinge and make a loop for clipping to a double ender. The cover protects from nicks & bangs (and makes a nice handle). I thought the little hinge would rust out after a year, but we've had the same ones for over 4 years. @Doc turned me on to them. Great conversation starter on a dive boat. Folks always ask me, "What's that tear drop thing clipped on your rig?"
Nice price, too
AdivingBel
Diver
