I am still rather new to AF-ON so I still have a question.
Do you ever user Continuous focus when on a tripod when shooting a subject that may be slightly moving within the frame? My understanding, if correct, is that with AF-ON and set to Continuous (AF-C) you need to hold the AF-ON button and it will lock the focus even if there is a slight movement of the subject within the number of active focus points. I have tried this hand held shooting technique on my dog who never sits still for too long. I focus on her eyes and use continous focus in case she moves her head. How would I do that with the camera on a tripod if the light is low and requires a slower shutter speed and hence a tripod?