In these two shots I positioned the subject ‘the girl’ on the path. Framing the portrait shot tightly with the same background from the previous exercise. I took two shots, the first image was at the longest focal length, the second I walked towards the subject to get the same frame but I zoomed out to my shortest focal length.
As you can see there are clear differences in the images. I used the bottom of the subjects t-shirt as the indicator for the baseline so the framing remained the same.
More of the background detail is surrounding the subject in the image with short focal length. It is the camera to subject distance that determines perspective, not the focal length.
If you were to stand at the same distance, the subject would appear exactly the same. So if you take a 50mm lens and an 100mm lens, there is no difference in perspective between the two, as long as you stand in the same spot and keep the subject to camera distance the same.There is an illusion of different perspective of lenses, because with long focal lengths you have to stand further away from the subject to frame them the same way.
The subject would certainly appear smaller with the 50mm lens due to shorter focal length and wider field of view, but the perspective and proportions would be the same on both.
So the point of longer focal length lenses in such cases, is the possibility to enlarge the subject in the frame, while keeping normal perspective.
I also took the same framed subject with my 50mm prime lens. This is a fixed focal length lens and as you can see the background softly out of focus.
A 50mm fixed focal length lens approximates the perspective distortion of human vision( not the angle of view, which is much wider). A standard lens is therefore the lens of choice for photography which aims to make an accurate record of the visual world as the eye would see it.