Perspective has nothing to do with focal length. It's determined by position of the eye (or camera) and the relative distances of various objects one is looking at.
The whole "human eye equals 40mm/45mm/50mm" thing is a misunderstanding of mixing up
magnifications when lenses were combined with typical viewfinders during the SLR era and comparing focal lengths to 135 format diagonal measurements.
If I look through a camera viewfinder with my right eye and leave my left eye open I will see an object in front of me with both eyes. If the apparent sizes are the same for both eyes, we would say the lens system (consisting of the total combination of elements in the camera lens as well as the mirror, viewscreen/focusing screen, prism, and eyepiece elements in the viewfinder) to be a magnification of 1X. If the object looks twice as large with my right eye, we would say the magnification is 2X. If the object looks half as large as seen with the right eye via the viewfinder then we would say the magnification is 0.5X.
Now let's discuss the viewfinders in typical SLR cameras. How large something appears when viewed in a camera's eyepiece depends on two factors:
- The focal length of the lens. This affects the size of objects as they are projected on the camera's focusing screen (sometimes also called a viewscreen) as well as projected on the camera's imaging medium. Since the mirrors in every SLR I have ever seen are flat, they provide no magnification as they flip the image up onto the focusing screen. The same is true of the pentaprism or pentamirror in the viewfinder. Since all of the reflecting surfaces are flat they provide no magnification.
- The magnification of the eyepiece. The lenses in a camera's eyepiece are very much like the lenses in a telescope or binocular eyepiece. They provide a magnification, usually a fractional one (that is they make things smaller), and project collimated light through the exit pupil. Our eyes then focus on this collimated light to view the image through the eyepiece. The size of the cylinder (or rectangle) of collimated light projected by the eyepiece is called the exit pupil size.
Many, if not most, 35mm SLR cameras during the second half of the 20th century had viewfinders that provided magnification similar to each other. With a 55-60mm lens attached the apparent magnification was about 1X. That means what we saw through the viewfinder with our right eye was approximately the same size as what we saw with our unaided left eye looking directly at the same scene.
The following cameras listed with their viewfinder magnifications with a 50mm lens focused at infinity:
Canon F1 - 0.8X,
Nikon F - 0.8X,
Canon AE-1 - 0.86X,
Minolta X-570 - 0.9X,
Pentax K2 - 0.88X,
Pentax ME-F - 0.87X. A 0.9X viewfinder would give 1X apparent magnification at roughly 55mm, a 0.8X viewfinder would do so at roughly 62mm.
In the digital age that standardization has been severely altered. Cameras have a wide variety of sensor sizes. Viewfinder sizes vary more from camera to camera. In the manual focus only portion (which was most) of the film era even lower priced cameras needed large, bright viewfinders to enable their users to focus them properly. With the advent of autofocus large bright viewfinders have become more of a luxury than a necessity and are seen mostly on the more expensive models. Differences in sensor sizes affect how much magnification is needed for the viewfinder to display approximately the same field of view as the FoV the imaging sensor will capture. It can take anywhere from 50mm to 70mm to get 1X viewfinder magnification, depending on the camera, sensor size, and viewfinder size.
All of Canon's FF cameras with 0.71X VF magnification require a 70mm lens to give the same magnification of distant objects as the naked eye sees.
If you're using a 5D, 5D Mark II, III, or IV, 5Ds/5Ds R, or a 6D/6D Mark II, then your perception that a 70mm lens makes an object look the same size in the viewfinder as it looks to your naked eye, you are spot on!