A mid-telephoto (85mm to 135mm) is advisable.

The reason is simple: It prevents distortion (widening of a face). Longer lenses create a 'compression effect'.

As to distance, well, it depends on what type of portrait you are creating. A close-up means a short distance, a full length a longer one, so there is not 'best distance at play'.

Note that portrait prime lenses are made to be at their sharpest at a set aperture and set distance. Test your lens to learn what it is, or ask folks who have the lens.

A mid-telephoto (85mm to 135mm) is advisable.

The reason is simple: It prevents distortion (widening of a face). Longer lenses create a 'compression effect'.

As to distance, well, it depends on what type of portrait you are creating. A close-up means a short distance, a full length a longer one, so there is not 'best distance at play'.

Note that portrait prime lenses are made to be at their sharpest at a set aperture and set distance. Test your lens to learn what it is, or ask folks who have the lens.

The prevention of distortion is a major issue in traditional portraiture. Distortion can impair the production of a good likeness which is one of the basic elements of portraiture.

It is important to understand that perspective is controlled by a distance not the focal length of the lens in use. Most folks consider focal lengh the most important factor and there is a valid point for that reasoning. So, we need to select a focal length that will enable a proper working distance for the composition, pose and head size we are incorporating in any given portrat image. I can't specify one exact working distance because that will depend on your composition, pose and/o, use of negative space, etc.

I can go into a lengthy post in that the term "headshot" can be a head and shoulder pose. a tight shot, not including the shoulders, and of course there are adults, adolescents, and little kids with differet head sizes. For general "headshot" work with a full-frame digital camera, my favorite is the 105mm lens which will enable a good working distance for most compositions in a standard headshot. Of course, there are equivalents in the other formats.

My recommendation is based on the fact that at a normal working distance to accommodate a well-balanced composition and pose, there will be no excess compression, exaggeration of facial features or structure, and no excessive compression of the background. The physical workg distance will not crowd or intimidate the subject but will be close enough to establish good communication between the photographer and the subject.

There are many other approaches, including the use of moderate wide-angle lenses that will enable more of the environment to be rendered sharply on a 3/4 length portrait, however, this requires specialized posing and camera positioning techniques. Longer focal length may be employed to bring about more compression (via distance) or less depth of field for background management, but again these are specialized techniques.

I do use a variety of focal lengths for portraiture but if I had only one les for that purpose it would be the 105mm- it would be the ideal lens for many shots and a good compromise for others.