PHYSICAL SUN AND SKY WITH FG
This will be a running journal of mental ray's new physical sun and sky in maya 8.5.
In Maya 8.5 , go to your render settings and in the mental ray tab under environment you´ll find Physical Sun and Sky - hit create ( also make sure you enable FG )
* What happens when you hit the Create button? A directional light is created with a mia_physicalsun attached to it's mental ray light shader slot. This light will mimic the sun and yep, to the point where when you rotate the light the light will adjust to obey physical sunlight. Also, a mia_physicalsky is attached to the environment of your camera, simulating sky and atmosphere. Lastly a lens shader named mia_exposure_simple is connected to your camera.
By default you´ll get a very accurate rendering of a daylight scene
*To add clouds to your sky, plug a spherical environment map into the haze parameter of mia_physicalsky
*Sky Textures http://www.1000skies.com/