One's point of view determines perspective with the camera and lens or the eye and brain.
Changing the focal length or sensor size changes the crop of the image.
In my experience on other sites, what trips up so many people is that for many casual photographers
the crop affects where they choose to shoot from, which in turn determines the perspective. I personally used to do this often before I learned better, and it's still something I need to watch. I think decades of shooting slides encouraged me to shoot this way, since cropping of slides intended for projection is not simple in the way that digital cropping is.
So the focal length, on a practical basis, can affect the perspective
indirectly. That
shouldn't actually happen, because the photographer
should understand all this and first
choose a perspective before choosing a focal length. If using primes, better to crop some afterwards and lose some resolution than to have a full-resolution perspective that doesn't do what you wanted it to do.
One thing that's tricky is that sometimes you
can change perspective modestly to 'fill the frame' to an appropriate amount without altering the perspective in an undesirable manner, while other times you really do need to stand in a certain place to shoot a particular image you want even if the intended composition doesn't fill the frame. Or you may want or need the initial photo out of the camera to be complete as is, for instance if quickly passing on a jpeg to others.
