The best option is to find a hole in the literature. Suppose you are interested in Ottoman history and roughly in Ottoman history between 1400-1600. Sometimes the people who write books on history point at holes that might be remedied by future research. Much as psychologists and neuroscientists like saying 'more research is needed', we say this. Suppose, then, one of the Ottoman historians say there was a Polish mission to the Ottoman capital at a particular time and it seems important for some reason, but he could not give much attention to it. If you can find the relevant documents, if they have not yet been studied (or not yet studied well enough), and if you do think it might be worth looking at, voilà.

An alternative path I often find myself on: there is quite a lot of work on the field, but you still think they miss certain things. Maybe they do not make use of some available document which is good for contextualisation; maybe their interpretation does not really follow from their evidence; maybe you suspect they were biased, etc. Or even, maybe there is new evidence that was not available to previous historians and using it you think you can come up with a new and better interpretation. This is a more risky approach of choosing what to study since you quite literally challenge previous colleagues; so I would suggest anyone considering doing this to be absolutely sure about their claims before proceeding. Take a step wrong or go too aggressive, you will be seen as just another young lion raging for recognition.

Perspectives matter in a different sense too. Different currents of historiography stem from the fact that a new generation of historians accuse older generations of writing history from the perspective of certain groups. It's a bit like *Battlestar Galactica'*s motto if you ask me: 'all of this has happened before, all of this will happen again'. So political historians accused of writing the history of politics as if it were all realpolitik an no culture/feelings/ideology by intellectual historians => those accused of writing the history of elites alone by social historians => those accused of writing histories that project their own elite/Western mentalities on the disadvantaged by cultural and 'subalternist' historians => accused of relativism and of denying the importance of politics by political historians... The cyclicality is, of course, a bit of hyperbole, but you get the point. Questioning of methodological underpinnings and presuppositions often lead to new interpretations of subjects studied for decades such as the colonisation of India by the British.

How historians choose the period, the era, the subject in which they are interested? I suppose that depends. It is not a secret and nothing shameful that we sometimes derive our research topics from daily problems. So long as we respect the usual rules of scholarship this is totally fine. Some people like chivalries when young so they study Medieval history, others do it because they want to see if Marx was really right about the feudal society. Some people study Italian fascism because they are shocked that something like this could happen, others write on it because they are curious how far our perceptions of fascism really corresponds to fascism as it was.