The Panasonic HPX300 can use different lenses via its interchangeable lens mount. It is a 1/3″ chip in a B12 mount, for which there are only a handful of lenses made by Canon and Fujinon. For an additional lens choice, JVC/Fujinon makes the ACM-17. It’s an adapter to mount a 2/3″ B4 mount lens onto a B12 mount camera (like the HPX300).
This allows you to use the myriad of professional glass available in the popular B4 mount. Note that the adapter is essentially a simple mechanical one. So, the camera will just be looking at a center cut out of the lens, which makes the result more telephoto than the same lens on a 2/3″ sensor camera. For comparison, a Canon 11×4.7 (4.7mm-52mm) will look like a 14-154 when mounted on the HPX300.
OK, this is where many people get confused. The lens is still a 4.7-52. A 4.7-52 on a 1/3″ sensor is what 14-154 looks like on a 2/3″ sensor. Make better sense now?
To get the same wide angle field of view as a 4.7mm on a 2/3″ sensor (about a 90 degree horizontal FOV), you need a 1.7mm on a 1/3″ sensor.