Why are certain lenses called ‘portrait’ lenses? Because its all about the perspective!
Classic portraits portray the best version of the person and part of that is not exaggerating features. We have a view of people that we normally see. Features looks normal at this distance and generally the person looks like the ‘normal self’. When you change your distance to the subject, your perspective changes. Features, such as a nose or lips, become more exaggerated. It’s not the focal length itself that changes your perspective, but the fact that you have to move closer or further back to keep the same composition with a different focal length.
To demonstrate this, here’s a series of pictures (the first half of which use the lovely Nikkor 70-200mm f/2.8 VR II on loan from our program) to demonstrate the fact. This is on a crop sensor camera, so the classic portrait range starts around 50mm but can be pushed to 35mm. These start from 200mm and end down to 18mm. See how the nose gets more exaggerated slowly and by the end is enormous! Each shot is framed approximately the same. I zoomed out and took a step in to keep the framing the same.
At this point, you can see the nose is clearly more prominent than at 200mm. Is it a problem though? Not at all.
Up until this point, the features look pretty standard, nothing major happening.
The features on the front of the face are clearly larger here! We’ve moved outside the ‘portrait range’ and it’s evident why the range exists!
So the change in perspective becomes pretty obvious by the end and you can see why the 50mm is the standard ‘start’ of the portrait lens length on crop sensor bodies. The features start to distort shortly after it. You could use 35mm on a crop sensor body in a pinch though.