My lens fogged the moment I stepped outside and I missed it

The light was right for four minutes. You knew it was coming, you were ready, you stepped out of the air-conditioned building and the front element went completely opaque.

It took six minutes to clear. The light had gone by minute three.

What happened

Your lens was at room temperature — say 68°F, because the building was cool. Outside, the air is warm and humid, with a dew point of 74°F.

Any surface below 74°F collects water from that air. Your lens was six degrees under it, so it started condensing the instant it was exposed, and it kept condensing until the glass warmed past that threshold.

Nothing was wrong with the equipment. It was simply the coldest thing in the vicinity.

Why it takes so long

Glass and metal have real thermal mass, and a lens is a solid lump of both. Warming it by six degrees using nothing but still, warm air takes minutes, not seconds.

That last row is the one to care about. External fog is an inconvenience; moisture that has condensed between elements or inside the body takes far longer to clear and can leave residue on internal glass or reach electronics.

The fix, which starts twenty minutes earlier

Seal the gear in a bag while it is still in the cool room. A large freezer bag is fine, a dry bag is better. The point is to trap the cool, dry indoor air with it.

Carry the sealed bag outside and leave it. The bag's outer surface goes below the outdoor dew point and condensation forms on the plastic — which is exactly where you want it.

Open it only once the gear has warmed. Ten to twenty minutes for most kit, longer for large lenses.

This is the only method that reliably works, and it is the standard practice for a reason.

If you have no bag and no time

• Get it into moving air. Airflow warms the glass faster than still air and carries away the saturated layer against the surface.
• Body heat. Under a jacket, against you, is faster than in a bag on the ground.
• Leave the lens cap off. Trapped air in front of the element slows equalisation.
• Do not wipe repeatedly. It re-fogs in seconds and you are dragging a cloth across wet glass that may have grit on it.

Going the other way

The same problem, reversed. Coming indoors from a cold shoot in winter, your gear is now the cold object meeting warm humid indoor air — and it will fog, and moisture can get inside.

Same fix: bag it outside, before you go in.

Knowing before you leave

The threshold is simply the outdoor dew point. If your gear is above it, nothing happens. If it is below it, everything below it will fog.

So the check is: what temperature is my equipment at, and what is the dew point outside? On a muggy day with an air-conditioned building behind you, that gap is routinely ten degrees or more — and it is entirely predictable before you pick up the bag.