Fogproof | Optics Trade Debates

By Polona Kraner Jul 31, 2018

Welcome to Optics Trade debates. In each episode, we talk about a different topic and try to answer the most common questions we receive about it. Today, we are going to talk about the term fogproof.

Fogging occurs when optics experience a rapid change of temperature. If there’s air on the inside of the optics that contains moisture, internal condensation will occur. The moisture starts forming small fog droplets inside the optics. So if the optics aren’t fogproof, they will not be usable in winter or in other situations where they would encounter substantial temperature differences.

Fogproof optics contain no moisture that could cause fogging. This is usually achieved by replacing the air with argon or nitrogen.

All premium riflescopes, tube red dot sights, binoculars, and monoculars are fogproof. Nitrogen is used in most cases and the use of argon is quite rare. All optical products that are fogproof are also waterproof.

The only types of optics that can’t be made fogproof are extendable monoculars and spotting scopes.

Argon atoms are bigger than nitrogen molecules, so it’s harder for them to escape. If there is a normal sized leak, both gasses would escape and air would replace them. Both work the same and there isn’t a „better“ one, because they don’t affect optical properties.

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Explanation of the term on our website:

Fogging in optical products can occur when you move them from the warm insides of your house to the cold outdoors. To prevent the formation of inside fogging they’re often filled with dry gas – either nitrogen or argon which contain no moisture.

To keep the gas intact on the inside, the optics have to be properly sealed, which is why all fogproof optical products are also waterproof.

It’s important to keep in mind that fogproof means that it’s to prevent fogging on the inside of the optics, not on the outside. If your outside surface of the lenses fogs up due to temperature differences or humidity just allow them to adjust back – do not wipe the condensation off as it can be damaging to the glass surface and its coatings.