Bird watching binoculars such as the center focus binoculars are extremely vulnerable to dirt and water; these can easily get into the focusing apparatus even though this could depend on their prism design. He trouble with dirty binoculars is that you won’t get crisp colors or sharp details; you need to learn a few important tips for keeping them clean and crisp:
Keep all the metallic parts clean and ensure that you use a wad of lens cleaning to tissue to carefully brush the lenses clean; you may also use a soft camel hair brush to remove any particle of sand or grit from the lenses. Failure to remove these and they could easily scratch the lens and its coating especially during the cleaning process; your binoculars should be held upside down so any dirt will fall away from the surface of the lens.
In order to prevent oil from your fingers from soaking through the lens cleaning tissue to the lens surface, ensure that you fold it, at least, four layers thick, always clean using circular movements as you gently wipe all the surfaces of the lens. If you discover an oil film, a single drop of lens cleaner on a tissue using circular wiping movements is enough to remove it. Hold the binoculars up to light and look into the objective lens for signs of dirt in the internal optics; if you discover any, don’t try any do-it-yourself cleaning but take it to a professional for internal cleaning.