I'm shooting with a D90 equipped with either a 70-200mm 2.8 or a 50mm 1.8 depending on the subject. My images are coming out blurry EVERY SINGLE TIME. It's driving me nuts. I know that the focus point at 1.8-2.8 is tiny...so what f-stop should I use when doing general portraiture to get the desired blurry back ground as well as a larger focus point (when shooting groups or from a distance? HELP!!! I want SHARP CLEAR images for my customers!!!