Depth cue renders fast. No good for God rays, but for fading the background, I agree with icprncss2. The trick, I think, to the depth cue is to moderate its intensity. For me, the longer the distance between the start and end point, the thinner the density. So if I start with a distance of 5m and end with 350m, it's much more subtle than if I start with 5m and end with 20m.

Another possibility is to add a water plane -- with a good BB water shader -- to meet the horizon line. It's frequently very convincing. You can usually justify a little strip of water in the distance in almost any landscape. :)

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.