Trick seems to be to render to the alpha channels instead of rgb channels... it's 8bit per color channel vs. 24Bit alpha channel apparently:

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ello: The trick is to render to depthmap to the alpha channel and not the rgb channel. The alpha channel is 24Bit whereas the rgb channels are 8 bit each. You might want to have a look at this paper:
http://ati.amd.com/developer/gdc/Scheuermann_DepthOfField.pdf />

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Cheers