A week or so ago I stumbled upon an online video where the speaker gave, at rather exasperating length, a tip for improved low light focus with the Fujifilm X100s.
I can’t find the link now, but will give a brief summation of his findings.
The tip: if you usually use AF-S but in a low light situation are having trouble acquiring focus, try switching temporarily to AF-C, as this will probably improve your ability to acquire focus.
Extra bits which support the tip, but which you don’t really need to read: the X100s has a ‘continuous autofocus’ mode setting (i.e. AF-C – which displays with a cross hair, rather than a focus box). This continuous autofocus setting doesn’t appear to work so well nor in quite the same manner as in other cameras with similar modes.
However, if you use this mode (AF-C) and focus on a subject, using it in the same way as you would when using the ‘single autofocus’ AF-S mode, then in low light situations it would seem to acquire focus faster and more often. Specifically, it would seem to be able to acquire focus in situations where the ‘single’ AF-S setting fails to do so.
The caveat: every time I’ve experimented with this tip, like right now, I haven’t actually had so much trouble acquiring focus in the ‘single’ AF-S setting anyway, so I don’t really know how effective this tip is. However, there have been instances where I have contrived things so as to be unable to acquire focus with the AF-S setting and in those same instances I could often, but not always, acquire focus using AF-C mode. So, give it a whirl – nothing to lose.
Addendum: found it again, here.