Fog and mental ray
 

Anyone know how to simulate fog with mental ray render. It doesnt recognize post effects fog.

Load the MR shaders(I forget the exact line but it's added to your Maya.env file). Then, on whatever camera you are using to render, in it's attributes under the mental ray shader, add a connection to the volume shader and under the mental ray tab there should be a a volume shader you can use. This is the closest I've gotten to actual fog in MR. Let me know if you have any other questions.