Water Torture

The left side of this lens is coated with Zeiss's Lotu­Tec. Notice how the water beads as compared to the untreated side. This beading action allows water droplets to slide off the lens for a clearer image.

Over the years, to make binoculars and riflescopes more rugged manufacturers have used improved adhesives and mechanisms to strengthen mountings of lens groups and adjustments. They've replaced spider-web reticles with etched glass and have found ways to fashion one-piece tubes and seal in nitrogen and argon gas to prevent fogging.

Then, after decades of working inside the scope, they took another look at the outside. "At Bushnell, we concluded that fog-proofing is only half the battle," says Bill Cross, who has been designing optical systems for many years. "The other problem is fogging outside, from your breath and sudden changes in temperature. Rain and snow obscure your vision, too."

So in the mid-1990s Bushnell began offering RainGuard, an exterior lens coating so slippery that water can't easily adhere to it. It beads up and runs off.

"Look closely at even high-quality lenses through a powerful microscope," says Bill, "and you'll see tiny pits. These enable water to cling. Our hydrophobic RainGuard coating fills the pits and leaves a surface on which the water just can't get traction. It's also a scratch-resistant surface because grit and hard objects can't find a place to imbed. They slide right off."

A third generation of RainGuard now appears on Bushnell's best binoculars and riflescopes. RainGuard HD is also oleophobic, meaning it sheds oils too.

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Another effective water-repellent coating is LotuTec, offered by Zeiss on its high-end Victory series. The name derives from the "lotus flower effect," which Zeiss explains as the hydrophobic characteristic of surfaces that encourage water to bead up and run off.

"LotuTec increases the contact angle of water droplets," says Zeiss engineer Walter Schwab. "Instead of sitting on the lens in a dome, a droplet pearls up in a sphere. Its base is smaller than its diameter, its height greater than that of a domed droplet. Because it has little purchase on the coated lens, it moves off.

"Because it takes the form of birdshot, water remaining on the lens has minimal effect on your sight picture. The distortion imposed by dome-shaped drops and subsequent water stains on uncoated lenses is much more disturbing."

Actually, LotuTec is three coatings that work in concert to prevent mechanical damage as well as shed water and oils. It has performed exceedingly well in my field tests.

StormCoat is a Burris lens treatment, introduced on Xtreme Tactical riflescopes in 2008 and now on its way into Euro Diamond and Black Diamond hunting models. According to Patrick Beckett at Burris, it is very effective in shedding water and fending off the grit from your shirttail.

"A lot of damage to scope lenses comes at the hands of their owners," he says. "In the field, we're all tempted to wipe dust or rain stains off a lens with whatever is handy. Repeated rubbing with abrasives harms glass. StormCoat resists scratches but doesn't affect brightness."

About a decade ago, Burris offered Q-Coat, a very durable film that also shed water but cost two to three times what the lens itself did and put the retail price of Q-Coated scopes out of the reach of most hunters. By contrast, Beckett says, StormCoat will become increasingly available on top-end Burris riflescopes without boosting their prices.

Patrick allows that while StormCoat and similar treatments are worthwhile options, they're not a panacea.

"We still use the wipers on automobile windshields coated with Rain-X--which, by the way, has figured into the development of scope lens coatings," he says.

He emphasizes that scope lens covers offer the best protection from the elements. "They also ensure that you get all the resolution and brightness you pay for," he says. "Even a thin film of dust on an outside lens will impair your view--especially if you must aim toward the sun. A cheap scope with a clean lens will outperform the most expensive scope with rain stains or dust on its glass."

Are there liabilities to water-repellent coatings? Not really, though they may be a bit more vulnerable to gun-cleaning solvents--which you'd best be keeping off lenses anyway.

How do you test optics you might wish to buy but whose descriptions make only vague reference to water resistance? Carry a small bottle of clean water with an eye-dropper cap. A single drop should bead and scoot off the glass if you tip it. Or squeeze a shot of fine mist from a spray bottle. The water should not streak but bead up like shot. If you breathe on a cold lens, the fog should quickly become tiny pellets.