Does anyone know how can I change my camera's focal length interactively, in the 3d viewport, (while looking through the camera)? I'm not looking for the "lock camera to view" feature, just so you know! I'm looking for the real zoom of the camera change/perspective "distortion"...
Right click that focal length button in the Shift + F7 Properties and set up one f-curve, then in the Shift + F6 Graph Editor, set up your desired lens shift (vertical scale) vs time (horizontal scale).
You could have a wide angle 15mm & 150mm telephoto on just 2 frames, (flipping between them with the arrow keys); or have a whole focal length transition that you play forwards (Alt + A) or backwards (Shift + Alt + A).
Difficult to stop that 2nd setup looping though.
It's fun to watch the (real) camera contorting lengthwise through one of the virtual cameras (Numpad 1,Numpad 3,Numpad 7 etc). It's also useful for controlling the size of the fly-mode 'dock', which has a freekish dependence on the active real camera's focal length.