Not wrong that you have heard it; but it is wrong that a 50mm lens is not a good portrait lens.
There is no such thing as “A Portrait Lens”
The Portrait Photographer firstly takes into account the PERSPECTIVE that is required to fulfil the vision of the photograph. PERSPECTIVE is determined by CAMERA VIEWPOINT. Camera Viewpoint is a combination of the DISTANCE to the SUBEJCT and the ELEVATION relative to the SUBJECT.
Once the PERSPECTIVE is determined then the FOCAL LENGTH of the lens determines THE SHOT – be it: a Full Length Shot; an Half Shot; a Tight Bust Shot; a Very Broad Shot . . . etc.
As already alluded in Posts #2 and #5, the PERSPECTIVE that is chosen will have an effect on the rendition of the FACIAL FEATURES: however that does not disqualify a 50mm lens being used on 135 format camera to make Very Tight Head Shots.
Obviously all photography, as an art form, is open to Subjective Critique apropos Artistic Value, but suffice to say this Portrait (below) has been awarded two Honourable Mentions and it was made with Available Light using an EF 50 F/1.4 on an EOS 5D:
I use a 50mm Prime Lens quite often for Portraiture. But, I use a 35mm Prime Lens more often, than a 50mm Prime Lens for that purpose. (on 135 Format Cameras)
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The equivalent of a 50mm Lens (an 80mm Lens) was the staple for most Wedding and Portrait Photographers using Medium Format Camera for many years – often being the ONLY lens which was used throughout the entire coverage and also for the Studio Portrait sessions. One skill that is necessary for that issue is to be able to carefully assess the camera's ELEVATION - DEPENDENT UPON ‘The Shot’ - to ensure the rendition of the Facial Features (and also Hips and Bust) would be absolutely acceptable to the Client - especially the Bride, who often were and still are the most critical of the single images which made of themselves.
As one example: a Photographer who knew the Craft well would use the Waist Level finder of a ‘blad or a TLR for the Full Length Shot but for the Half Shot the camera would usually be raised to about Chin Level.
For a Tight Head Shot, for example to display the Diadem, it was not unusual to see Photographers shooting with the camera sideways to the Bride and the Waist Level Finder at eye level, or some short photographers would need to shoot with the camera held inverted.
Many of the Portraits, across a the range of Broad Shots to Tight Shots, made in that era by Photographers who knew their Craft and using only that one 80mm lens are extreme example of fine workmanship; so obviously a 50mm lens can be used to make quality Portraits today, there is absolutely no reason why it cannot be so.
WW