Internal fogging can be a serious headache for birdwatchers, or anyone else who uses binoculars. Fogging on the inside of the binoculars interferes with the viewing, and can't be easily wiped away like external fogging can. The moisture on the inside of the binoculars that causes fogging can also damage the binoculars, by promoting the growth of fungus inside them. Fortunately, the treatment of internal fogging is a fairly simple procedure, though it requires patience.
Leave the binoculars in a warm dry place for several days. Most binoculars are not airtight, so any moisture inside the binoculars should evaporate into the drier air outside.
Seal the binoculars inside an airtight plastic bag along with some commercial desiccant. The desiccant will absorb the excess moisture, eliminating the source of the fogging.
Keep the binoculars as dry as possible on future outings, once the moisture has been removed. If you are using the binoculars during wet weather, keep them covered when you are not looking through them. Wipe off any outside moisture as soon as possible.
It is best to store binoculars in a dry area even if you have not noticed any fogging. If your binoculars are supposed to be airtight, and they have not been damaged, they may be defective. If fogging occurs with airtight binoculars, check to see if there is a warranty, and if possible get the seller to replace them. Avoid touching the lenses of the binoculars with your fingers, as the oils from your skin can cause smudging.
Do not dismantle the binoculars unless you have sufficient expertise to put them back together. Dismantling the binoculars may misalign the lenses, alter the optics and ruin the binoculars.