Modern (clean!) microfiber cloths are the best tool for cleaning the optics. Usually, fingerprints, for example, can be removed by breathing on the lens elements and wiping them with a light, circular motion. It is not recommended to use dry paper cleaning cloths and a fine powder as offered in many optical cleaning sets.
Dust can be removed with a lens brush or bellows which can be found at any photo store. Before cleaning with a cloth, dust, grains of sand, and other hard particles must be removed carefully.
Dried rain drops and primarily salt spray leave deposits and rings. They must be removed by moistening or (with watertight binoculars) with water before a cleaning cloth can be used.
Oil or weapon grease on the lens elements should be removed as quickly as possible. The lens elements should be cleaned carefully to the edge using an eyeglass cleanser or an optical cleaning cloth and perhaps a little alcohol.
For normal dirt, watertight binoculars, or a dismounted riflescope can be simply rinsed with lukewarm water.
Alcohol, acetone, cleansers
T* multi-coated lenses can be cleaned with pure alcohol. Acetone, random cleanser, or benzene is generally not recommended. They can corrode and damage various materials and coatings.
Watertight, dry nitrogen-filled devices are not subject to mold build-up – at least on the inside. Mold can grow on the exterior lens elements if the device is stored in a moist, warm and dark environment for longer periods. Mold spores are practically everywhere and fingerprints (i.e. grease stains) provide an ideal breeding ground. In such cases or in the tropics, where high humidity is the rule of thumb it is recommended to store the devices in an airy, bright location. If they are warmed by the sun, any moisture on the surface vaporizes; without moisture there is no mold buildup. Mold often marks its presence with a fine, branching structure on the surface of the lens elements. The problem is that metabolic byproducts damage the surface permanently so that a simple cleaning may remove the mold but not its traces.
Behavior in winter
Following a winter excursion in the snow, cold binoculars or a riflescope are best stored in a dry, cool room. In a warm room they would fog (exterior) immediately.