shoot wide open at 2.8f.
>shoot at shutter priortiy - set to 800 or 1000 at least.
>take time to bump ISO slowly after a couple of shots.
Hey, you know this makes no sense at all... right? You can't lock down shutter speed and your f-stop unless your shooting manual. You give up control of the f-stop when you shoot shutter priority. Your last statement though it the golden one where you nail it. Find your exposure, move to manual, and then change ISO when you need to adjust exposure. Only a few cameras allow you to lock both shutter and aperature while having the camera adjust automatically ISO.