The first thing to do is remove the dust and dirt that has adhered itself to the lens. Use a brushing tool for this. It is a special brush, much like a thick artists brush, often with a rubber air blower on the end. Gently brush the lens while squeezing the rubber end. This will dislodge the tiny dust particles that have collected there. Do not use too much pressure as there may be harder particles present that can scratch the lens. That would be much worse than the dirt itself. The bottom line is to be careful.
In most cases this will be all you have to do. Stopping the cleaning process right then and there won’t expose your camera in further danger. You never want to put your camera in further danger of damage. However, there are also oils that will build up on the camera lens that require more than brushing to remove. For this use a clean, soft, high quality cloth. There are also special cleaning tissues available from camera retailers. They come in different quantities. If you use real cloth, make sure you wash it between uses. It only takes on tiny left over dirt particle to permanently scratch the lens.
There are two types of liquid cleaners for cleaning camera lenses. If the film on your camera lens is water soluble, you can use distilled water available at your local grocers. The second type is with you as you read this. It’s your breath. Using your warm breath on the lens creates a type of distilled moisture. The point is that the liquid must be clean and free of harmful minerals. If you use distilled water, pour a very small amount onto your cleaning cloth. Never, ever pour it directly onto the camera or lens. The water could easily seep inside the camera and permanently damage it. Always err on the side of caution. It’s better to repeat the cleaning process with too little moisture, than to have too much water on the lens.
There may be oils present that require more aggressive cleaning. For this type of grime you’ll need regular alcohol. Be sure that it is 100% pure alcohol. Anything with additives could damage the lens. Don’t use things such as paint thinner or toxic liquids. The reason for this is that the camera is close to your face. Camera retailers also sell special camera lens cleaners. These are good for lenses that are saturated with oil based film. Follow the same procedures as with the other liquids.
All cameras, from that old Browning film camera you got as a kid, to large Single Lens Reflexes and today’s Digital SLR Cameras have lenses. They all require cleaning at some point in time. I cannot over-emphasize the importance of using due care when it comes to cleaning your camera. That sophisticated piece of equipment was an expensive investment.