The BCC Z-Blur filter uses a Z-map image to emulate a rack focus effect.
Move the focal plane through the source image, using the Z-map to control the focus. Adjust the focal point, depth of field and blur parameters to finetune the area of the image to blur.
You can also set a channel from the image clip, then use that channel to control the z-blur effect. This type of effect is most noticeable in real life with a long focal lens, such as a 105mm or greater zoom lens.
The built in PixelChooser, with integrated mocha planar tracking allows for greater control over which areas of the image the filter should affect.