PetaPixel pointed out a Nikon support thread that specifically says not to use your breath:
Do not breathe on the lens to fog it for cleaning. There are harmful acids in breath that can damage lens coatings.
Uh oh. Then what the hell are we non-professional photographers supposed to do? Nikon says lens cleaning solutions are the best (no abrasives, no solvents) but if you don’t have that, use “the blower bulb, then brush, and wipe the lens in a circular spiral from the centre outward.” The more you know, right. [Nikon via PetaPixel]





















I have Hoya UV filters that live on the front of the lenses. No worries blowing on those and wiping on my shirt, if I can't find a lens cloth.
Filters more often than not tend to decrease the image quality by a LOT. Reason being, lenses aren't calibrated to produced the best images with an added piece of glass up front. No matter how perfect or invisible the glass in filter actually is, it will tend to distort the light, making the difference in quality quite noticeable.
I'm not convinced about this.
Money, money.