Not everyone wants to hear it, but the classic rule of thumb for proper perspective on portraits (so noses don't appear enlarged, etc) has always been to stand back 6 or 8 feet or more, and then zoom in as desired for framing. So for waist up or head and shoulders or head shots, 50 mm will be too short to allow that proper distance. Standing too close messes up perspective (enlarges noses, etc).
For classic 35mm film cameras, 105mm was always considered a great lens choice for "portrait" (because it forced that distance). That equivalent would be 70mm or 65mm for 1.5x or 1.6x cropped sensors... and the longer lens can have less depth of field too, easier bokeh if that has to be the choice.
But there is no one answer possible, until "portrait" is defined.
Head shot?
Head and shoulders?
waist up?
full length standing?
A group? of 4? Or 44?
And of course, is it a full frame or cropped sensor? They have different fields of view.
The one answer is to just always stand back at least 6 or 8 feet, and then the right lens is the one that will frame the view that you want. It won't be the same lens in all cases (but zooms can work well).
Some think portrait means f/1.8. But that's the last thing I want, I like f/8.
But for minimum depth of field, a longer lens is normally a better choice than an excessively wide aperture (and its lens problems). For one thing, the longer lens only shows a narrow background, so just shifting camera position slightly can better choose the background detail to be shown, or to be excluded.
