Reviving an age-old thread because .. Well this may be old hat but I just found out you can use the Focus Distance Guide to measure where the start and stop in the depth cue should be.

 

And it makes such an interesting fog effect, especially if you plug a turbulence node into the Depth Cue colour and play a little with the settings :)

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.