Portrait lens is perhaps one of the most debated topics in photography community because it is a topic of really no definitive answer. There will be many, more than many swear that how should portrait be made and therefore by which lens, some may go further to theorize portraiture photography base on masterpieces. And that is where the problem came from.
Not that masterpieces are non-exist, on the contrary, there are lots of masterpieces, of many kinds, by many people, of many styles. Similarly, the development of photography is hugely tool related, from the early day large sheet film technical camera to smaller roll film camera to modern day digital, people’s way of taking picture and the approach is constantly developed, mostly for good.
Is there any lens can rightfully called a portrait lens, I do not believe. Slightly change in distance between subject and the photographer has profound impact on the perspective of the image and human brain is so delicate that our conscious sense the smallest detail and feel, and exactly that is why so many photographer s eventually use different tool and approach to produce each own craft. Yes, there are textbook stuffs while ironically, textbook is for students.
The image for example here, on the beautiful Taiwanese model Sandra, shot with Sinar Hy6 + Schneider 80/2.8 AF-D, Sinar emotion 75LV. Lighting with Bron-Color Verso A4 with Para 220FB.