Originally posted by tabl10s without the assist light?
I just tried at two different exposures.
* ISO100, f/2, 30s - struggles a bit, but finds focus after a few tries. If I'm not mistaken that is roughly EV -3 - which in turn is about as much light as a landscape reflects under a full moon and no artificial light around.
* ISO100, f/2, 5s - locks focus quite effortlessly. This is still less than EV 0 - sitting in a very poorly lit room. If you shoot handheld you'll end up with something like f/2 and 1/60th at ISO25600 - which is
noisy - but it will focus.