Originally posted by timo Hhhmmm - there have been long, laborious threads here and on dpreview demonstrating that neither focal length nor angle or view are related to perspective, strictly defined. Perspective depends entirely on the relative physical positioning of you (the photographer) and the various objects in front of you; that will not change no matter what lens you put on the front of the camera. (Distortion is something different.)
you are absolutely right about that, thanks for clarifying.
Consider the following:
(A) You want to fill the frame with head and shoulders from your subject.
(1) Using a 50mm eq. / 40° H-AoV. (Horizontal Angle of View)
(2) Using a 28mm equivalent / 65° H-AoV.
While keeping (A) premise, you will notice the the perspective difference becouse you changed lenses! (Ok, it is becouse you got closer to your subject to keep (A) but lets not be naive. This is what we think of, and not about distance or perpendicular plane to our subject).
Expanding the normal lens concept, it should have a focal length which allows the observer to view pretty similar through the camera as he sees with the naked eye.
This actually sounds quite lazy but it points out the "subjetive" matter in the definition of what is normal to someone.
Let me re-state:
"In 135 format (24x36mm), the diagonal of the sensor has 43mm. so, using a FL
arround 43mm
assures will produce an image with normal perspective; does not compress the edges or expand the center,
while mantaining your relative position to the subject when seeing with your naked eye"
I think the 50mm design is fairly simple with great optical qualities. Maybe this is why almost every 28mm looks good on APS-C (it equals the sensor diagonal) but that´s another topic.