I have the Nikon Digital SLR D70s, having a hard time choosing a lens for portrait/fashion/editorial photography. I want to achieve a crisp sharp image of the person, but blurred background. i am a little confused, i keep reading that the longer the lens the better DOP you get for portraits. BUT it seems that all the longer lenses have higher aperature rates. Would it be better for me to get a shorter lens (the 50mm) with a lower aperature than the longer with the higher aperature, or vice versa??