It also depends on how much of the body is showing in the 'portrait'. A head and shoulders portrait is best captured, as has been mentioned already, in the 85-200 (FF) range. I use my 70-200mm almost exclusively for this. However, if you're doing half to full length, I'll switch to my 24-70mm or even the 50mm prime. Most of the full length portraits I shoot are easily done with the 50mm. For more environmental portraits where the subject is full length and less than half of the image area, you can use just about any lens depending on the effect you want. Google Al Gilbert's environmental portraits using a wide-angle lens.