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Nuke 7 added a new tool to create defocus effects on to your images, zDefocus. This video goes over:
-The difference between a defocus and a blur
-Older tools in Nuke that create defocus effects: Convolve and Defocus
-The new zDefocus tool.
-How would you apply this tool to a image that has no imbedded depth information
-How would you apply this tool to a 3D render that has depth information.
You can download the EXR file I used here: