How to Lens Whack / Free Lens using a DSLR camera
Lens whacking is the filmmaking method of fully disconnecting your lens from your camera body, and moving it side to side, and backwards and forward from the camera sensor to achieve lens flares and artistic focus. This is because instead of light just entering the camera sensor from through the lens, light is also coming in from behind the lens where you have disconnected it from the body.
If you are lens whacking you are going to want to make sure that the lens you have chosen to use is 50mm focal length or less, any more and your image will be shaky as you will be holding the lens freely in your hand.
The best way to hold your camera and lens whilst doing this trick is to have your camera body in the palm of your left hand, with the bottom of your lens being supported by your fingers. With your right hand you want to hold your thumb over the record button as you normally would, and your fingers covering the gab behind the lens. By doing this you can adjust how much light you are letting into the lens, reducing or increasing the amount of lens flare within the shot.
For the camera settings, you need to make sure your shutter speed is double your frame rate, this is normal to any kind of shooting. Make sure your focus is set to infinity as you will not be needing to adjust focus whilst whacking with the lens.
Below is a lens whacking test that I made to show the effect that you can achieve. You can see the purple lens flares which I made by allowing light to enter from behind the lens, also it turns my Canon EF 50mm f/1.8 II lens into a macro lens. The further you pull the lens away from the camera body, the closer you can focus to the subject, this is how I shot this macro video of the beer bottle.
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