Here's a typical scene from a tourist destination, usually somewhere in the southern US: an older guy wearing sandals, dress socks and a guayaberas shirt, carrying around a Nikon or Leica camera outfit and taking snapshots at high noon.  After every few shots, he takes out his handkerchief or lens tissue and cleans the front element of his lens, removing all dust and any possible smudges.  Sound right?  Take a hard look next time if you don't know what I'm talking about!!  But more importantly, does this do any good?  Will dirt and dust really show up in pictures?  Is it necessary to clean minor dust and fingerprint smudges from the camera lens?
 
I've spent many sleepless nights contemplating just the thoughts above.  Now, thanks to some unscientific testing and serious abuse, I've been able to sleep better at nights with my new found knowledge.