All David Hobby wanted to do was pass along photo lighting tips to a friend. He jotted down some ideas and posted a blog —Strobist— to show how any beginning photographer could get beyond ordinary shots by using inexpensive, $300 flashes, just like pros. Then the world found out about him. Hobby now has 200,000 mostly amateur photographers as devoted readers. His site attracts 1.6 million page views a month.