How can you stop a scope fogging up?

Q: When I travel to my stalking grounds in a warm car I get scope fogging on my lenses in the cold weather. What, if anything, can 
I do to stop this? It’s a real nuisance and very irritating when I want to get out in the field. 

(Read deer stalking clothing – this is what to wear)

Bruce Potts offers some solutions to scope fogging

A: It can indeed be a real pain 
when you get to your shooting grounds after a long drive and then, when you remove the rifle from its 
case, the scope’s lenses fog over 
with condensation in the colder air.

If it is just the external surfaces that 
are fogging or showing signs of condensation, it is not too bad. (Read how to choose a riflescope.)

Putting a cold scope into a warm car can cause fogging

A bigger problem

It is when scope fogging happens internally 
that you have a bigger problem on your hands. This means that the scope’s seals have failed and the inert gas used to purge the scope to keep it dry has escaped. This will require repair 
by the manufacturer.

The likely cause

However, it is more likely that the rifle is warm from the car and it is simply the change in temperature causing the problem. To prevent this, keep the rifle in the case but put it in the coldest part of the car — for example, the boot — and then when you arrive at the shoot bring it out and unzip it and let it acclimatise a while to the new temperature. Also open the scope’s 
flip-up covers if it has them.

Be patient

It is best to wait until the condensation does not form any more, as you know what will happen — a deer will walk out and you won’t be able to see it, and it is dangerous too. I also always have a small scope cloth in a pouch that attaches to the sling so I can keep the scope clear if necessary as I stalk.

Early morning mist

Q: I have noticed that my scope tends to become misty when I go stalking in the mornings. I can’t seem to get rid of it and the scope fogging seems to be getting worse. What can I do?

A: If the misting is on the outside of the lenses, it’s condensation, which is common at this time of year when a rifle is removed from a warm car or case and meets cold air. However, it sounds like an internal problem. This is called fogging and is due to a failure in the internal sealing system that lets moisture enter the scope, hence the fine water particles, misting, on the lenses.

Most scopes are purged with an inert gas, such as argon or nitrogen, that eliminates any moisture and the scope is also sealed with O-rings and/or the lenses are cemented. I suggest that at least one of the seals has failed. Usually, it’s at the eyepiece end of the scope where a variable power scope or focus ring is used regularly and therefore wear occurs and a leak happens.

This will require a return to the manufacturer, I’m afraid. If it happens in the field and does not cover too much of the lens, you can place both hands around the scope at the affected area and the warmth from your hands can reduce the misting.