Planet 5D blog post: blog.planet5d.com/2011/11/getting-rid-of-flicker-in-timelapse-the-lens-twist-method/
Aperture flicker is the biggest enemy of DSLR time lapse enthusiast. The lens twist method is how you defeat it.
For most camera controlled lenses the aperture closes with each shutter release. Because of a DSLR's inability to open and close the aperture the exact same amount EVERY time there can be slight variations in exposure from picture to picture. These variations create the unsightly problem that is called aperture flicker. This procedure locks the aperture, so that there's no variation between actuations.
Update: Doesn't work with Nikons :(
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