Many things can happen to low-quality optics in the cold. However, the best cold-weather rifle scopes will not be damaged by cold temperatures.
Extremely cold temperatures can cause some materials, such as metal screws and springs, to become brittle and some parts to seize as lubricants freeze. In high-quality scopes, however, these temperature-related material changes usually require much more extreme temperatures than, for example, those experienced in a truck during a cold night. Although cold temperatures will probably not damage your rifle scope, the cold can affect your rifle scope in other ways.
Leaving a Rifle Scope in Cold Weather Overnight Might Not Damage It, But Warming It Too Quickly Might
Riflescopes have parts made from many different materials, including plastics, metals, and glass. As materials experience changes in temperature, they expand and contract. This expansion and contraction can create both thermal and physical stresses within the material.
1. Thermal Stresses Can Crack Lenses
Most of us have had seen a hot glass object, such as a plate fresh from the oven, shatter when it comes into contact with something cold, such as a cold countertop or cold water. The plate shatters as a result of thermal stress. Specifically, the surface of the plate wants to contract because it has suddenly cooled, while the interior of the plate wants to expand because it is still hot. This imbalance in thermal stresses causes the plate to shatter.
Similarly, moving a rifle scope out of a warm truck and into cold air might cause the lenses to crack. This is particularly true if the weather is windy or rainy since water and moving air will cause the temperature of the lenses to drop more quickly. In this respect, it is better to allow scopes to adjust to extremely cold weather gradually by leaving them in a protected place, like your truck, overnight. The gradual temperature drop is less likely to create thermal stresses than a rapid temperature drop created by taking scopes from a warm place into the cold outside air.
Cracked lenses can also result from the reverse situation when the cold lenses of a rifle scope are warmed too rapidly. Trying to warm up a rifle scope by placing it in the hot air blowing out of the truck’s heater can introduce thermal stresses into the lenses, causing them to crack.
2. Physical Stresses Can Warp Parts
Different materials have different thermal properties. This means that they heat up and cool down at different rates and expand and contract differently as their temperatures change.
Rapid changes in temperatures can cause physical stresses as certain parts contract or expand faster than other parts. For example, metal expands and contracts more than glass. If a warm scope is cooled rapidly, the metal barrel will contract more quickly than the lenses and could crack them. Conversely, if a cold scope is heated rapidly, the metal barrel will expand more quickly than the lenses and could cause misalignment of the lenses or gaps to develop between the barrel and the lenses. These physical stresses might be reduced by allowing rifle scopes to change temperature gradually.
Moisture, Not Cold, Is the Enemy
The problem with gaps between the barrel and lenses of a rifle scope is that moisture will enter the scope. Moisture causes fogging and allows corrosion and mold to develop inside the scope. Moisture, therefore, causes many more problems for the long-term usability of your rifle scopes than cold temperatures will.
This is where purging comes in. High-quality rifle scopes are usually filled with inert gas, such as nitrogen or argon, rather than air. Air carries dust and moisture, both of which can affect the image viewed through the scope. Pushing the air out of the scope body with nitrogen gas is intended to eliminate this problem.
The quality of the scope will become apparent in cold weather. A low-quality scope may allow ambient air, along with the dust and humidity carried by the ambient air, to enter the scope’s body. The result is fogging, frost, and dust on the inside of the lenses. The best cold weather scopes, on the other hand, are sealed against interior fog, frost, and dust.
Cold Weather Fogging Can Be Prevented with Lens Coatings
Fogging on the outside of the lenses, however, can occur with any scope regardless of quality. Rather than reflecting the quality of the scope, fogging reflects the quality of the lens coatings.
Scope lenses fog when moisture condenses on the lenses. This occurs when the temperature of the lens is lower than the dew point of the air. Cold air holds less moisture than warm or hot air, so it is unlikely that any fog on scope lenses in very cold weather has condensed from the ambient air. Rather, your own body gives off moisture through perspiration and respiration. This can cause lenses to fog or even frost if you breathe on them or hold them close to your body. This is more of an issue with spotting scopes, range finders, and binoculars than rifle scopes since those devices are used with the eyepiece held in contact with the face.
One way to prevent fogging is to breathe away from the eyepiece and keep your un-gloved hands off the eyepiece. Additionally, lens coatings can prevent moisture from sticking to their surfaces. For example, Bushnell’s® EXO Barrier™ lens coating fills in the imperfections of the lens surface to create a fog and debris-resistant surface.
When It Comes to Cold, Quality Matters
The good news is that the best cold weather rifle scopes are designed with temperature-induced stresses in mind. Rubber seals, advanced adhesives, and other technologies can secure lenses while protecting them from physical stresses caused by the scope’s barrel and insulating them from thermal stresses from heating or cooling too fast.
Moreover, high-quality seals can prevent moisture from entering the scope when changes in temperature cause the scope barrel and lenses to expand at different rates. This will prevent fog and frost from developing on the inside of the scope. Fog and frost on the outside of the scope lenses can be minimized with lens coatings and careful handling and use of the rifle scope.
Bushnell® leverages its years of experience and precision engineering to produce some of the finest cold weather optics in the industry. View the full range of rifle scopes to pick the one you will use this winter.