Focal length wise, well a 50mm lens on a full-frame sensor camera (or traditional 35mm cameras) is roughly equal to human vision. On a cropped-sensor DSLR, 50mm is actually closer to 80mm. This is fine for portraits. However, there are many reasons for using a longer focal length. For example, if you shoot someone with a larger nose from further away using a longer focal length, you compress things somewhat. (Shooting someone close up with a wider-angle lens makes even the smallest button nose comically huge.) So, the focal length decision depends on what is most flattering (or not) for the subject, how much room you have, and the effect you are trying to achieve.
Hope that helps.
Have fun.
