I was reading about lens whacking on the web so I decided to go give it a go. All of the camera effects you see my video above were done in-camera. Lens whacking created the selective focus effects and the light leaks, which gives an overall soft dreamy and vintage effect.
Lens whacking or freelensing is done by removing the lens from your camera and holding the lens away slightly from the camera body. It is best to use a lens different that the make than your camera, I shot my video with a canon t3i and an older Nikon 35mm f2 lens. I set my camera and lens to manual, infinity focus and the widest possible aperture. During the day the aperture was not that wide, maybe f8, to prevent over exposure but as it got darker I was able to open my aperture wider and get more specific selective focus
The effect is achieved by moving the lens around holding it slightly away from moving the camera. Moving the lens slowly side to side changes what area is in focus and moving the lens further away from the camera creates the light leaks. It is not that easy to control what the camera chooses to focus on, so I mostly just stayed with it and tried different things out until I liked what the lens was doing. The best thing to do is experiment until you get a feel for how the lens behaves with the different movements, by the end of the day I was mostly able to choose my focus points and light leak levels
Camera shake is also a big thing with this as you are holding the lens with just your fingers and moving it around, no way around that really, but I did use motion stabilizer in Final Cut pro in some shots.
Obviously, you are leaving your sensor open to dust and dirt, I didn’t notice anything on my camera, but it’s a risk you have to be open too.
This technique can also be used with still photography in the same way to get a dreamy/lightleaky/vintage/ tilt shift type effect in camera.
Video and still images taken with a canon t3i and a Nikon 35mm f2 lens