Just a quick test to see how portraits look at vastly different focal lengths. I used two lenses for this starting off with the 24-120mm for the wider to "normal" portrait pics and then the 70-300mm for the two longer focal length pics. I tried out the following focal lengths: 24mm, 35mm, 50mm, 85mm, 120mm, 135mm & 200mm.
I would like to say at this point that I don't think there is any right or wrong in which focal length you prefer to use for portraits, it really is up to you and your own vision. Don't let anyone try to tell you different.
As you can see from the series of photos below the wider the focal
length the more distorted the face and also more background. The longer
the focal length the more compressed the face and background become.
I prefer fairly standard focal lengths for my own portraits, more specifically the 85mm for headshots and 50mm for half or full length portraits as well as environmental portraits if I have the space, otherwise the 35mm is great for environmental portraits as well.
The choice at the end of the day depends on vision, so everything goes because it's an art, not a science.