Perhaps this will help de-mystify things:
- Focal length is an optical property of a lens. It is the distance from the primary nodal point to the imaging plane with the lens focused to infinity. Has nothing to do with a format, with a field of view, or with perspective.
- Field of View is the result of combining a particular focal length with a particular format, presuming the lens' image circle is larger than the format. 35mm Film has a format size of 24x36mm, FourThirds cameras have a format size of 13x17.3mm. Since the diagonal of 35mm Film format is roughly twice the length of the diagonal of FourThirds format, the same focal length lens fitted to FourThirds presents a Field of View which is cut in half by comparison. This is what makes a 50mm lens fitted to a FourThirds camera seem to look like a 100mm lens fitted to a 35mm Film camera.
- Perspective is the relationship of the apparent relative size of objects in a scene. It is governed entirely by the distances from the camera to the objects in the scene. Neither focal length nor field of view directly influence perspective ... the field of view defined by a focal length and format simply allow more or less of the scene to be apparent in an image capture.
So, an example:
If you put a 50mm lens on a 35mm Film camera and frame a scene, you establish a "reference scene" with a particular field of view and perspective.
Fitting the same 50mm lens on a FourThirds camera you will see a scene which has one quarter the area (1/2 the angle of view in both horizontal and vertical dimensions) that has the same perspective as the reference scene. If you move the FourThirds camera back far enough to include the reference scene, you will change the perspective due to the change in distance.
If you change the lens on the FourThirds camera to have a focal length of 25mm instead of 50mm and position the camera in the same place as before with the 35mm Film camera, you will reproduce the reference scene, with the same perspective.
In any case, the other optical characteristics of a particular lens, like the focusing distance range made possible by its lens mount and the amount of light gathering power it is capable of made possible by its aperture, do not change. They are invariant across the formats a lens' image circle can cover.