Books to screen

Over the years there have been great movies that originally came from great stories written by authors, who no one has ever heard of, and sometimes from popular authors, However, some movies do not provide the same magic as their counterparts and leave quite a bit of material out of the story. This sometimes upsets the audience and the author, because of the changes made in the story to make the movie more enjoyable for the audience. Is this fair to the author, and fans of the story?