Three challenges: One, the syncing of the accelerometer reads with the exposure - while the exposure itself might take a small fraction of a second, it will be hard to match that up with accel data - leaving you with covering the whole duration of the API call which can be a source for false alarms. Two, rotation. The up-down-left-right style movement can be judged, but you can get a blurry photo with all sensors reading 0 because the movement was angular. Depends on how you hold/press your camera I guess. Three... your subject might be moving in which case the right thing to do is to replicate the motion of the subject AFTER focus.