
Freelensing is shooting with your lens detached from your SLR body and holding it just in front of the opening. It's a cool technique to experiment with as no expensive additional equipment is required!
Freelensing can produce interesting effects such as: a very shallow depth of field, macro focussing and light leaks. For these examples I used my prime 35mm Nikon on a D5000, I also reversed the lens so that it was the wrong way around.
I've broken stages I approached this task below:
1. I made sure that I was in a dust free environment - it is much easier to get dust into your camera body and on the sensor by shooting this way.
2. I shot the subject normally with the lens attached in aperture priority mode F1.8. I noted the shutter speed used. The focus was set at the minimum distance.
3. Changed to manual mode and set the aperture to F1.8 again and the previous shutter speed.
4. Removed the lens and held it in reverse directly against the body. I had to hold the small lever on the back of the lens (or front in this case) to open the aperture up. On my lenses, when you detach them this lever springs in one direction and closes the apeture to the minimum setting.
5. I focussed by moving the camera closer or further away from the subject.
6. ...and then Shot!
Further Reading:
Freelensing on flickr:
http://www.flickr.com/photos/lukeroberts/sets/72157608917928435/
On Photojojo:
http://content.photojojo.com/tutorials/tilt-shift-and-macro-freelensing/
101 examples:
http://gizmodo.com/5679403/101-photos-taken-with-the-lens-detached