Filched from another site ... and apologies to TEd.

There was this haunted house on the outskirts of the town which was avoided by all the townfolk - the ghost which `lived' there was feared by all. However, an enterprising journalist decided to get the scoop of the day by photographing the fearsome phantom.
When he entered the house, armed with only his camera, the ghost descended upon him, clanking chains etc.

He told the ghost "I mean no harm - I just want your photograph".

The ghost was quite happy at this chance to make the headlines - he posed for a number of ghostly shots.

The happy journalist rushed back to his dark room, and began developing the photos. Unfortunately, they turned out to be black and underexposed.

The moral of the story - The spirit was willing but the flash was weak.