I'm a little bit confused about the First Pass (described on pp. 15–16 of the rules). Is there a Lens and a Focus as usual, or does this section proceed without a Focus? If there's no Focus, how do you know when the First Pass ends and the rest of the game begins?
No. The rules don't suggest that you do, and it would be contrary to the purpose of the first pass: to flesh out the history a little bit before you begin normal play. If you had a Focus, then the starting history would be far too narrowly focused to be a useful start.
You know when the first pass ends because everyone has made one card and placed it (p. 15, emphasis mine):
Each player now gets to add more detail to the history, creating either a new Period or Event. Players can go in any order they want.
Note also that there is no turn order during the first pass – you just make a bit of history each and place them as players come up with them. The first pass should really be quite quick. When you're done the first pass, there should be a number of cards equal to the number of players plus two for the bookend Periods.