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The Private, Exclusive Guide for Serious Divers Since 1975
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April 2009    Download the Entire Issue (PDF) Available to the Public Vol. 35, No. 4   RSS Feed for Undercurrent Issues
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Get A Magnifying Glass

from the April, 2009 issue of Undercurrent   Subscribe Now

When I was a new diver, I was out for the big stuff. But after seeing hundreds of barracuda and lobsters, I appreciated more diverse pleasures in the reef environment. Thatís where my magnifying glass comes in. The tiny shrimp lurking in the lettuce coral, the clinging crab in the anemone and the pipefish in the sand flats are easier to identify and a lot more spectacular with a good glass. I also get a lot more detail on other denizens, including the corals.

If you want to go small, you need a glass lens, not plastic. Thatís because the index of refraction of water is about 1.33. Plastic lenses have about the same index as water, so they become useless underwater. Optical glass has an index of about 1.52, so it works, though the magnification underwater will be a bit less than above.

Index of refraction? Thatís a measure of how much a substance slows the speed of light, and thus bends (refracts) it. Waterís index of 1.33 also explains why objects appear a third larger underwater -- and why your buddy swears that a three-foot grouper was a four-footer.

Magnifying lenses in your mask (bifocals) are not very useful for viewing underwater subjects except gauges. This goes for the plastic stick-on bifocals as well as the Mares ESA mask, which has separate lens frames in which you can put magnifiers.

Instead, you need a good hand-held glass. My favorite is from Edmund Scientific ( Look for the Round Magnifier with 10X Spot Lens. The four-inch lens (you donít want a small one that you need to hold too close to the subject) is available there for $11.95, plus $7.50 for shipping. It fits in a BC pocket and has a hole in the handle for a lanyard. Happy hunting. - - M.A.

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