OP
idocsteve
Guest
I was going to say "Ask Jon about this one".
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
I suggest you confirm your "reader" diopter need in VERY dim light. It may be closer to +2.5.
Diopters are a metric unit of measure. Put a +1 on a camera lens that's focused at infinity and 1-meter is what's actually in focus; +2 w/lens @ infinity = 1/2 meter in focus; +3 = 1/3 of a meter in focus, etc.
Submerged, objects appear BOTH 33% bigger and 25% closer – both are true simultaneously (PADI's Encyclopedia of Diving gets the facts wrong - it's not "either," it's both at the same time). This magnification is why true angle-of-view gets so narrower underwater. Both magnification and lateral chromatic aberration is not uniform but gets worse as you look off-axis (i.e. away from where your nose is pointing):
To precisely compensate for underwater magnification, bifocal diopter is adjusted for that 25% reduction in distance (e.g. +2 in air = 500 mm in focus; reduce that distance by 25% = 375 mm, requiring 2.66 diopter; but ophthalmic lenses only come in .25 increments). Also, the "vertex" distance (cornea-to-inner-surface of lens) of a dive mask lens is further than a spectacle lens, which requires a reduction in a positive lens (like holding a magnifying glass further from your eye). Bifocal strength is HIGHLY subjective: the stronger the bifocal, the narrower the view, and arm extension can easily position objects for a best compromise position.
That's a very detailed technoidal way of saying:
1. Confirm what you need in VERY VERY VERY dim light above water
2. Add 0.25 diopter, unless...
3. If you're a photographer needing to confirm critical focus on an LCD screen, use +0.50 extra on one eye (each eye does not need to be the same strength because you don't see through them both at the same time anyway, per post #126).
Now, all that said, the "magic bifocal" phenomenon of dome optics is why hundreds of divers wear contact lenses to use some of the 9,000 double-dome masks we've shipped around the world since 2003. You get great close focus AND distance vision in the entire field-of-view (i.e. your eyes behave as they did when you were in your 20's).
idocsteve --my keratoconus OU is too steep and now having trouble tolerating "piggybacks" (i.g. combined soft and hard contact lenses).
Now considering phakic collamer Intra-Ocular-Lens Surgery:
Visian ICL Implant and Verisyse-Artisan Lens IOLs
Idocsteve,
How bad does it get? I've reached "that age" and my eyes are starting to get that aging thing happening.
I fried my eyes at university but only needed a +1 for the longest time and only when i was reading. In the last few years it's gone to +2 (right on schedule). Does that mean if I live to be 80 that I'll need a magnifying glass to see my own hands by the time I'm 60? .
the bad eye is now 20/35, down from 20/160. I am showing improvement, and only need correction for reading.
You'll be able to see your hands at 80 however you might not be able to count your fingers and even if you could you'd probably forget how many you had by the time you got to 5.
Acutally, some days I already have that... LOL
I was actually wondering though, in all seriousness, if the worsening of ones sight is something that slows down at some point or how that works.... ? any insights?
R..