John, as an eye doctor, I'm curious as to which Rx's this mask is good for ie hyperopia and myopia (& the amount of each) or just the latter.....also, how much astigmatism will/can it cover??....plus, what do you recommend for presbyopia when someone who is an emmotropic presbyope contacts you?...just curious, TIA.....
Hi diver 85,
First I digress, for everybody who's not sure what we're all about. Here's a link to a recent post on SB on the "
What did you buy that you regret" thread. THANK YOU to "shoupart" who happens to use our mask - NOT a paid shill.
OK - to answer your questions: Emmetropic presbyopes (for those who don't happen to be eye care pros, that's someone age 40+ with 20/20 distance-vision who needs reading glasses)... they would have to wear disposable contact lenses to make them a "temporary" myope. YES, that IS a hassle, but doing so achieves FAR superior accommodation (close-focus ability to you non eye docs!) than any above-water optical system, as Ill explain. (what we call the "magic bifocal" phenomenon).
To be a naked eye match requires myopia but unlike all above-water eyewear, in range that varies with age. We have an
Rx conversion chart on our website:
The goal is to avoid all eye strain and ensure that objects both near and far can be focused (accommodate in the eye doc vernacular). And dome optics makes everything much sharper (i.e. eliminates all
lateral chromatic aberrations):
Everybody's above-water vision Rx is measured accurate to within ¼ diopter, and that's how eyeglasses and contact lenses are dispensed. But when analyzing an underwater dome system with ray-trace analysis, Snells Law and our knowledge of physiology, TWO phenomenon are used to determine the correct Rx:
1. Young people naturally have greater accommodation (they can focus nearer-than-infinity vs. older people). And because each eye is concentric to each dome, there are no visual cues that the Rx is too strong. Conversely, with eyeglasses where ones eyes are not concentric to the geometry of the lens radii, a too-strong minus lens makes things (A) look smaller than true, (B) further than true, and (C) causes barrel distortion.
2. Dome optics actually CHANGE their diopter power underwater, depending upon the viewing distance to the target. Huh?! Because the object (e.g. fish) is swimming IN the lens (i.e. the water-column IS the lens), and as Snell taught, thick lenses (longer water-columns) are stronger than thin lenses:
Because of #1 (young eyes can comfortably focus closer than older eyes), divers under 30 can be between -2.5 and -6.0 eyeglass Rx and use the mask equally well. Age 40+ have to be between -4.0 and -6.0 eyeglass Rx, but your CYL has to be converted (details on our website).
RE how much CYL can be accommodated? Just as SPH-only contacts can correct to a 0.75 CYL no problem, and for many to a 1.0 CYL, the dome can be used the same way. Because there are few straight lines underwater, and CYL correction is somewhat subjective, we've heard from divers with 3.0 CYL who are happy without needing toric contact lenses. But being an optical perfectionist, we recommend up to 1.75 CYL ("over-SPH") before considering disposable toric contacts. But for divers with high CYL, they should first test the mask in a pool before resorting to torics.
Here's an
earlier ScubaBoard post that goes into more detail RE avoiding eye strain.
