Why is so much nonsense talked about telephoto compression?

Yes, that's what I thought he meant too.

The size of the hand compared to that of the face might be a clue.

There are suggestions that a different perspective was used for the portrait and the background:

nicofranz.art/en/leonardo-da-vinci/mona-lisa

In fact, it looks to me like some early photographic portraits done in studios using a painted background scene close behind the sitter.

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I am well aware of your intention, though an exaggerated perspective would have been remarkable for that era and out of character with Da Vinci. Nonetheless, as I wrote at the end of my reply, it's an interesting thought experiment.

Twenty-one years ago I wrote the following article:

"Perspective Compression by a Telephoto Lens: A Myth".

You can Google it. It's based on elementary optics theory and some simple math. You know --- SCIENCE. I wrote it because I got tired of the nonsense being spewed back then on this topic and hoped that it would clarify the issue. Things have improved since then but, as this thread demonstrates, the nonsense has not yet been eradicated.

Your article shows that the image produced by a lens of longer focal length is simply a magnified version of the image produced by a lens of shorter focal length.

Your article claims that perspective compression depends on the camera position, but not on the focal length.

Let's take a simple scenario. You want to take a photo of a long straight road with a lot of traffic on the road. You want the photo to show strong perspective compression.

Where should you stand? In what direction should you point the camera?

You take a FF shot with a 50mm lens (as you said that focal length does not matter). Will it show perspective compression?

Just to add a little more to what I said previously.

Your article is part of the nonsense talked about perspective compression. You claim that perspective is not affected by focal length because a photo taken with a longer focal length is simply a magnified version of a photo taken from the same position with a shorter focal length.

What you fail to appreciate is that it is that very magnification of the image that causes perspective compression!

Try looking through a telescope and you will see perspective compression without having to change your position. It is caused by the telescope magnifying the image.

Another way to describe it is in terms of viewing distance from the image. Moving closer has the same effect as magnifying the image and compressing the perspective. Artists who study perspective have known this for 600 years or more!

Compression may or may not be a bad term. However, there is some justification. To really understand what is going on one needs to go back to the lens equation from geometric optics. In geometric optics there are two types of magnification that are determined by the lens. There is lateral magnification (perpendicular to the optical axis and longitudinal magnification ( along the optical axis). One can easily show from the lens equation, that If one fixes an foreground element in the focal plane and then calculates what happens other objects of the same height behind the focal plane - that the magnification on those objects is a increasing function of focal length. That is if your have a person standing in front of a bridge or building and you frame the person to take be the same image height, then the height of objects behind the woman will be smaller with wide lenses and larger with longer lenses. The smaller image behind the person compared to the larger image of the same object will make the distance between the two objects with the wide angle scene expanded and the longer lens contracted. A common mathematical term for this type of transformation is a dilation in the case of the wide angle and compression in the case of the longer lens.

Clear is needed to not make the mistake of think that the geometric concepts of distance, dimension and angles are preserved in the projective transform a camera performs when it transforms a conical volume in three dimensions on to a plane. Projective transforms do not preserve any metric on three space.