It comes down to taste but if you like the front of an object to look about as far away as the back of the object then some people advise getting about 5 x away from the object as the object is big. So for a head and shoulders you want to be 5 head and shoulder lengths away. After that you need to select the lens that give you the framing you like. From that distance you start to need an 80 or above.
A 50 is “normal,” according to many, but to see a person only as a head and shoulders you would need to stand closer than most people stand to each other. That’s why the 50 is not a common choice for portraits. But again, it comes down to choice.
