How to Keep Your Lenses From Fogging

Why would you put holes in something that’s meant to be acting as a shield? Seems a little counter-intuitive, right?

For one very good reason: fogging.

Those who wear glasses are probably all too familiar with this. You step out of the cold and into a heated building, and whoosh! Instant fog. Or you’re grinding on a killer climb, sweating and heating up, you pause at the top to catch the view and bam! Fogged lenses.

Why exactly does this happen?

Two words: Phase transition.

Even if you’re not familiar with the term, you’re probably familiar with the concept. It’s a change from one state (solid, liquid, or gas) to another without a change in chemical composition. Most phase transitions come about through a change of temperature or pressure, which results in a change to the substance’s internal energy.

For example, if you freeze water (liquid), it will become ice (solid). If you were to boil that ice cube (solid), it would return to its liquid state and then turn to steam (gas).

The reason this happens is that heat always flows from things with a higher temperature to things with a lower temperature until they are the same temperature.

When you’re exerting yourself, your body is heating up, and then emitting that heat through sweat. Since heat is always looking to move to a home with a cooler temperature, it directs itself to your cooler lenses. As it cools, it loses the ability to hold its vapor form, and condenses on your lenses, causing them to fog.

The key to keeping your lenses from fogging up is to keep air circulating behind them. One way to do this is to use a larger nose piece so that your sunglasses aren’t sitting as close to your face. If you prefer a tighter fit, or if this alone isn’t enough, this is where vented lenses come in handy.

The small cuts in the lenses allow air to get back behind your lens, so the heat that you’re emitting doesn’t have a chance to settle on your lenses. Instead, it’s carried away on the wind. The shape and position of the cuts are designed to best address ventilation while still keeping debris, rain, and sunlight out of your eyes.

Of course, if you stop moving, the lenses will inevitably fog. There’s not much that can be done to ensure this will never happen. Many have tried, but there’s no such thing as a 100% success rate when it comes to keeping fog at bay. However, the vents in the lenses will ensure that once you do get moving, your vision will clear within seconds without you having to take off your sunglasses to wipe them clean.

Short of attaching mini fans to your sunglasses, vents are one of the best options for clear vision.