You're in a pinch and some kid who's never seen a camera before just grabbed yours, front element first, leaving a nice handprint just before your shot of the usually stern village chief lifting his kid into the air. There's no time to grab that lens cleaner in your bag, so what do you do? You open your mouth in an 'O' and breathe hot air on the front, and then rub your lens off gently with your shirt, right? Wrong.
Just recently, NikonRumors had an interesting comment from case in which Nikon Support said, "Do not breathe on the lens to fog it for cleaning. There are harmful acids in breath that can damage lens coatings..."
I'm sure it won't kill your lens to do it once, but there has to be a reason that Nikon is saying not to breathe on your lens. I know I've done it a few times before in a pinch, especially while traveling -- who hasn't? In any case, be careful. Now someone's officially warned us all...
Here's the entire text from Nikon Support:
How do I clean the camera lens?
The best way to clean a lens is to use a piece of lint free lens cleaning tissue and a small amount of Lens Cleaning solution. Do not use anything containing abrasives or solvents, only use Lens Cleaning Solution.
First we recommend taking a small blower brush to blow off or brush away loose dust or debris.
Next, place a drop or two of cleaner on the tissue (never directly onto the lens) and then wipe the lens in a circular motion, beginning in the center and working your way outward, removing any marks or smear.
If the above supplies are not available a clean, dry, soft, lint free cloth can be used to clean the lens. Do not breathe on the lens to fog it for cleaning. There are harmful acids in breath that can damage lens coatings. Just use the blower bulb, then brush, and wipe the lens in a circular spiral from the center outward.
The same method can be used to clean the viewfinder eyepiece of Nikon cameras.