Just like a human eye, camera lenses have a near point at which they cannot focus anymore. This focus distance depends on the lens construction, but typically wide angle lenses have nearer close focus distances than telephotos. Many macro lenses, of course, are optimized for near close focus distances.

Most of Canon’s new professional (L series) lenses introduced since mid 1999 are equipped with gaskets and rings to keep out dust and moisture. They’re not waterproof, by any means, so don’t go diving with them. But they’re much more resistant to inclement weather than Canon’s other products.