Delight in the Details

On my last trip to Cozumel, besides nurse sharks and turtles, I didn’t see big critters. But I saw an eye-popping painted elysia sea slug looking as fine as the one pictured in Paul Humann’s Reef Creatures.

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.

That’s 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 ID 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. That’s 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? That’s a measure of how much a substance slows the speed of light, and thus bends (refracts) it. Water’s index of 1.33 also explains why objects appear 1/3 larger underwater, and why your buddy swears that a 3-foot grouper was a 4-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. I’ve tried several and my favorite is Edmund Scientific (www.scientificsonline.com). Look for the Round Magnifier With 10X Spot Lens. The 4-inch lens (you don’t want a small one that you need to hold too close to the subject) is available there for $11.95, plus $6.40 for shipping. It fits in a BC pocket and has a hole in the handle for a lanyard. Happy hunting.

– M.A.