Waterproof binoculars and spotting scopes are sealed internally with O-rings to prevent moisture, dust, and debris from compromising the inside of the instrument. Fogproofing means that optical barrel (or barrels in the case of a binocular) are filled with nitrogen or argon gas to prevent internal fogging caused when subjecting the optic to temperature extremes, especially in high humidity. Fogproofing is sometimes referred to as nitrogen-purging. This term is probably more descriptive of the actual process; O2 (oxygen) is purged from the barrel(s) and replaced with an inert gas (usually nitrogen) that doesn’t condense.
A binocular or spotting scope can be waterproof without being fogproof. This merely means that the instrument is sealed, but the oxygen inside the barrels haven’t been purged and replaced with an inert gas. But if an optical instrument is fogproof, it has to be waterproof (sealed) as well. Fortunately today, even most inexpensive and moderately priced binoculars and scopes are both waterproof and fogproof.
Waterproof and fogproof binoculars (or scopes) are highly recommended if you plan on using your gear in rainforests, anywhere where the humidity is high, or if you’re around water (on a boat or near a wetland). Some of the best birding can be experienced during inclement weather and one of the main reasons birdwatchers go with binoculars that have both of these features. An optical device that becomes compromised by moisture will obstruct your view and may ultimately become damaged from mold growing on the inside of the barrel(s). Likewise, if you intend to use your optical gear in an extremely dry climate, the O-ring seal will prevent contamination from dust and debris. It’s well worth having the added confidence that your investment will not be damaged from exposure to adverse elements!
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