A bit of experimenting today. This photo is taken with a Panasonic m4/3 camera without a lens. Instead I used a 50mm Canon EF lens that wasn't physically attached to the camera. I just held it in front of the Panasonic's lens mount and focused by moving the canon lens closer or further away from the Panasonic lens mount. I had to use a small piece of cloth to cover the gap between the lens and the camera in order to get a proper exposure.
I think this technique is usually referred to as "freelensing".