What is a portrait lens?

Answer 1

A lens for taking pictures of people's faces. The best length is considered to be between 50 and 105mm (if you have a camera with a cropped sensor then your lens will be 1.6X longer so your 50mm would become about 78mm). These lenses have large apertures of about F2.8 to ensure good background blur so there are no distractions from the subject's face. ********

There was a time when zoom lenses were not of sufficient quality to be used for portrait work; certainly not for any serious professional, anyway. It is a generic term (which probably was coined even before that) because a portrait lens focal length depends on the film format being used. 80 mm was about right for 35 mm film, but 105 was OK, being slightly long. I disagree with the notion expressed above that anything around 50 is OK for 35 mm film; it's too short. As for 120/220 film, portrait begins at around 150 mm, so you can see what I mean by "portrait lens" being a generic term. Also, when using longer focal lengths, depth of field is less when all other factors are equal. Therefore, smaller f stops such as F8 are required, not larger, so I don't see the point about that either. For 35 mm, an 80 mm lens at F2.8 at ten feet provides a depth of field of approximately 9 inches. This is not enough to render the complete subject in focus (from one shoulder to the next) in a 45 degree pose.