With many binoculars, you will see Nitrogen Waterproof or Nitrogen Gas Filled. So what does this mean ?. Think of what happens in your car or house when its cold outside and warm inside. You get condensation on the inside of the window. The same happens with binoculars when you get extremes of temperature on the inside and outside of the binoculars.. This is simply the air turning moist and causing the misting or internal fogging.
With Nitrogen gas filled binoculars this does not happen like with air filled binoculars. This means they can be used in extreme temperatures and humidity without fear of internal fogging.
Nitrogen waterproof binoculars are not to be confused with water resistant binoculars that simply have a rubber seal at either end and are not fully waterproof
You can purchse nitrogen waterproof binoculars from http://shop.northernoptics.co.uk