How to Clean a Digital SLR Lens
Cleaning dSLR lenses is a lot less intimidating than cleaning your camera’s sensor. Cleaning lenses is a lot like cleaning windows. You should become comfortable cleaning lenses whether they are attached to the camera or not. You never know when you’ll need to clean one. Also, you will rarely need to clean the rear lens element (it’s normally covered by a cap or the camera).
If you must, use the same steps as you would to clean the front of the lens. In either case, if you spray a cleaner, spray it on a cloth, not the lens itself. This keeps fluid from seeping into spots where you can’t clean it.
Brush off the lens before you go rubbing anything on it.
Or, use canned air to dust the surface. This will hopefully remove anything that might scratch the lens as you try to clean it.
Use a damp (not sopping) microfiber cloth or unused coffee filter to clean; gently rub the lens from the center out.
That’s if you’re using a NIKKOR lens: Canon recommends wiping from the outside in, using a circular motion. (Think, Wax off.)
Occasionally, you may need to use a lens cleaning solution to remove oil. Nikon has a Lens Pen Cleaning System that you can also use. It's a combination brush and scrubber.
Make sure you’ve wiped off all cleaning fluid (water or otherwise).
The important points are to use a soft, clean cloth or lens cleaning tissue and only approved lens cleaning solutions — or none. Most lens manufacturers recommend against thinners or benzene because of the plastics involved in the lens body. Be careful using any sort of chemicals on lenses with coatings.