For normal cleaning, use the least aggressive method first: blow off loose dust, then gently wipe the outer surfaces with a clean microfiber cloth. Some users also use a small amount of mild dishwashing detergent when needed.

If the dust or marks are actually between the filter’s glass layers, there usually isn’t a practical way to clean that yourself. In most cases, minor internal dust or small imperfections will not noticeably affect image quality, so the best approach is often to leave it alone.

Also, avoid excessive cleaning. Repeated wiping increases the chance of scratching the filter surface, which can cause more visible problems such as flare than a few tiny internal specks ever would.