My theory regarding portrait perspective

I have very little practical experience in portraits but I have been reading tutorials online here and there whenever I can. My kids being very young do not seem to be willing participants so I think I will look into getting a manequin to practice.

One aspect of portraits that arises often in these online tutorials is that of the ideal perspective and many authors/bloggers differ in their recommendations. I have been thinking about this and came to the following conclusion:

The portrait should be taken at a distance that corresponds to the distance from where the photograph will be viewed, modified by the scale of the photograph to the subject.

So if I have a life sized photograph of a person and it will be seen in a gallery from 4 metres away, then the camera should be 4 metres away from the subject when the photo is taken.

First, I'd like to know if this makes sense and your thoughts. Second, I'd like to know what would be the distance of the camera (and what location of the camera specifically) to the subject versus the distance of the viewer (again, what location specifically? the retina?) to the photograph to get the identical perspective?

No, that theory won't work. Let's say you're shooting a group. Unless you intend to view the group from fifteen or twenty meters away, you won't be far enough back to "get them all in" with a normal focal length lens (28-30mm with a 15x22mm, 50mm with a 24x36mm camera), so you'd have to resort to a wide-angle lens. However, at that short distance, the exaggerated perspective of the wide angle lens will distort the appearance of the group members on the sides.

While there is a relationship between the appearance of exaggerated perspective in the image and viewing distance,* you don't have to go to all that much work. The first thumbrule is to be farther than two meters from a single subject with any focal length.

If you want to add some complexity to your calculations, make your distance ten times the depth of the subject to avoid exaggerated perspective with any lens. So a single subject face-on is 20-25cm in depth--2-2.5 meters would be sufficient distance even with a wide-angle lens. However, if the subject extends a hand toward you, that increases the depth to about 80cm, so you would have to be about 8 meters back to avoid exaggerated perspective with any lens.

If you need the edges of the frame to avoid exaggerated perspective, do not shoot with a lens shorter than the "normal" focal length, and if at all possible, no shorter than about 1.5x normal focal length to be sure.

Extreme distance introduces compressed perspective, but that doesn't become noticeable for most people until you're 15 meters or farther from the subject, and not usually objectionable until you're quite a bit farther away.

*If you view the image from a distance equal to the focal length of the lens, the perspective will always look natural. Yes, that would be a distance of 30mm from eyeball to print if you shot with a 30mm lens.

here's the sticky on perspective control
https://photography-on-the.net/forum/showthre​ad.php?t=672913

Ok, I understand how distance to subject affects perspective - i.e. it's related to the relative distances between the eye/sensor and different objects in the scene.

What I am saying is that in order for the perspective in a photograph to appear "normal" to the viewer, the angle of view of the subject must be identical both to the human eye observing the print/image and to the camera sensor during the capture of the subject.

So, for example, you can shoot a baseball team with a wide angle lens (e.g. 24mm) from close up which gives an diagonal AOV of about 84 degrees. Or you could shoot the same team with an 85mm lens giving a diagonal AOV of about 26 degrees. If both of these photographs were made into a 24x36" print, then the ideal viewing distance for each would be different, in order to get a 'natural perspective'.

For the 24mm photo, the viewer should be close enough that the AOV is 84 degrees. For the 85mm photo, the viewer should be at a distance to get a 26 degree AOV. In both these cases, the viewer would be closer to the photo than the camera was to the subject because the prints are less than life size. Were the prints a scale of 2:1, then the viewer should be further from the print than the camera was to the subject.

Basically, I am saying that the magnification of the final image should and the viewing distance (i.e. AOV) should be considered when taking the picture to duplicate for the viewer what the camera saw.

This is the part that is relevant. The angle of view is not relevant. They eye/mind happily accepts angles of view different from that of the human eye (which is about 160 degrees or so), but does not always accept unaccustomed perspectives caused by relative distances between objects in the image.

The eye/mind tolerance for unaccustomed perspective is much greater for diminished visible perspective than for exaggerated visible perspective.