Funny how people claim this focal length or the other to be classics for portraiture.
In my time, I have seen Leica claim that the 125 Hector(?) was perfect, then the 90mm with the M series was perfect.
I have used both 85, 105, 135 and 200mm for this job and in the end, it all boils down to what turns you on regarding perspective. For what itīs worth, my own feelings are this: 85 is too short; 105 is passable but can still induce some unpleasant distortion and 135 is as close to perfect for me as is possible, whilst 200 flattened perspective too much. As I have never used a 125 I canīt comment on that.
The thing is, how do you define portraits? It makes all the difference what you intend to include. My own preference is tight head-shots, which means that on FF 35mm Iīm around 5 to 5.5 ft away from the subject. Which is close enough to communicate yet not so close as to crowd the poor model.
On occasion, I have used a 50mm lens to take waist to top of head shots, but this has been something forced on me because of circumstances such as being in hand-held mode and not wanting to risk losing the mood by changing optics or using a tripod. (A zoom would have precluded the shot anyway, because as you end up carrying the weight and weight distribution of its longest focal length, it would have been unlikely that an off-tripod situation would have arisen.)
Itīs a funny thing, but even changing camera format can create its own set of preferred focal lengths which do not necessarily tie up with/ equate mathematically with what you would have selected on a different format.
I think a hell of a lot more goes on in the brain in this business than one can identify or even articulate.
Expanding the thread slightly, Iīd say that even choosing between film or digital makes a difference, even when within the same type of body shape. That difference, I feel, is that digital encourages one to shoot a lot of rubbish which the film discipline would have stopped in its tracks. Not so sure that such a waste of finger actions is ever a good thing. Though I had motor drives on all my Nikons other than the very first, the F, I think I remember only one time when I used any form of continuous firing. - their purpose was simply to do the winding on, leaving me free to concentrate on the picture without risking prising my eye out of my head with my thumb.
Ciao - Rob C