"Fogging" occurs when an object is cooler than the ambient temperature and humidity is high... It's condensation forming on the cooler surface. If it's happening visibly inside your lenses, it's likely happeing on metal surfaces inside the camera too. How likely it is to occur depends upon how wide the temperature differences and the ambient humidity.
The solution is two-fold:
First, put your gear inside a sealed bag of some sort for 15 minutes or half an hour (maybe longer), giving it time to adjust to the ambient air temperature. Do this when going to and from air conditioned and non-air conditioned conditions, for example. I used to do it a lot, too, when out shooting in cold winters. After working outside in the cold, before taking the gear inside a warm room I'd double bag it and seal it up securely... Leaving it sealed for half an hour to an hour, just to be safe.
Second, reduce the ambient humidity around your gear with a dessicant (such as silica). It's available in loose form, but you'll want to keep the dust away from your gear, so instead get it in some sort of "breathable" package. There are single-use packets (such as you find in retail packages of all sorts). And there are reusable packs that often have an indicator to alert you when the silica is saturated. You then put it in an oven on low temp for an hour or two, to dry it out so that it can be reused.
You can use most any sort of plastic bag or sealed case for these purposes. And the desiccant packs are widely available. In a particularly humid environnent and/or wide ranging temperature differences I might put one or two in my camera bag, another in any cabinet or closet where the gear is stored and one or two inside the plastic bag or case being used to transfer the gear bewteen cold/warm conditions.
I don't think it makes any difference based upon lens quality. All lenses have air inside them and some means of "breathing". Even the best sealed lenses have to breathe. At best, "lens sealing" makes them dust and moisure "resistant"... not completely waterproof or dust proof.
Thankfully, where I live now there's rarely very high humidity and the temperature ranges aren't all that radical.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
