Peter bargh explains the art of lens wacking with a Pentax digital SLR.
The idea is you allow more stray light to reach the sensor and to do this you shoot with the lens detached from and held in front of the camera body. The technique has been used by movie makers using their video modes on DLRs for a few years and has found its way into stills.
It's a fairly tricky technique to master as the infinity point is lost and you have to shoot at closer range. You also have to control how much light spills in through the open lens mount, while holding the camera and lens with one hand and triggering the shutter with the other.
The careful balance is to get light to spill in at certain points creating large patches of low contrast while keeping parts of the subject sharp. As a result it's a really good technique for flower photography and portraits.
The choice of lens is crucial. I find old manual lenses are good. A wide-angle lens of 28mm restricts the distance you can move the lens, so it will be too close to the camera body, while a 100mm has to be held further away and too much flare can enter the camera. The 35mm to 50mm range is ideal, and as you don't have to have the lens coupled. Any 50mm will do, from the cheapy Chinon's / Zenit's to the super quality 50mm f/1.4 AF Pentax. I used an old SMC Pentax-M 50mm f/1.4.
Set the lens to infinity to gain maximum range and move the lens too and from the body to focus
Hold the lens between your thumb and middle finger while gripping the camera with your other hand. Use your index finger to make the variable lens shield. Move it around to block or allow light though.
As there is no exposure coupling you need to experiment with settings. I used manual and set the lens fully open to deliver large bokeh and shallow depth of field. I then set a guessed shutter speed, checked results and adjusted again until the exposure was correct.
You can also tilt or shift the lens for interesting effects the out of focus bokeh becomes distorted when you tilt so you see ovals rather than circles.
A word of warning - the insides of the camera are exposed so dust is more likely to land on the sensor. There's nothing you can do about this other than taping a clear filter or plastic over the mount to cover it up.
The shot below is a straight shot taken with the Pentax K20D and an old SMC Pentax-M 50mm f/1.4 attached.
Here's what happens with the same location, when the lens is detached and held in front of the camera. The camera was pointed up from a slightly lower angle to ensure the sun from above hits the lens mount on the body and bounces some flare around - lens wacking! Notice the slightly mishaped bokeh (out of focus highlight circles) as the lens wasn't held totally parallel top the CCD.
Another photo with the 28mm f/2.8. A wider angle is harder to use and doesn't work quite so well.