AnandTech Guide to Better Photos: Portraits
All lenses can be use for portrait, however the depth of field distortion (compression/expansion) look more natural between 35~100mm (for 35mm cameras). Closedown aperture will produce (DOF) Bokeh result on all lenses. However the distance from camera to subject is too close for comfort for normal or wide angle lens, therefore photographer tend to use 70~100mm for portrait taking with out too much of a depth of field compression. Long telephoto lenses also are use by portrait & fashion industry to bring the background more to the subject or to crop unwanted background, and bokeh effect also can be employ in this lens class to focus the attention on the subject.
Suggestion on not using direct lighting is good, but there are such thing as diffuser that will help in such a situation (produce more pleasant image than bounce or in situation that there is no wall or ceiling to bounce off). Slow sync flash also is a good suggestion, however indoor lighting will throw off the natural human skin colour.
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