Legal Question in Technology Law in California

If your photography is done from a place where you have a right to be, in most cases it is not illegal to photograph anyone. Search [legal rights photographers].

It is not illegal to photograph someone whether they want you to or not. However, it is either illegal (in the sense of contrary to statute) or actionable in a civil suit for damages, or sometimes both, to do any of a number of other things that the photographer might do in addition to pressing the shutter release button. These include trespassing, stalking, invasion of privacy, etc.