Fog for background

[Mushir Qazi]

Hi people.
There is a query and i am sure people are here going to solve that.
Well coming to the point there is a animation of a building in which there are lot of low poly buildings in background to give glimpse of a city.
I am attaching couple of images for better understanding one is my test render and another one is kind of fog effect I am aiming for.
In the attached image camera will be moving towards building.
I heard that there is a method to achieve with z depth but there is another question arise.
i.e In Z depth when I set min and max value at the very first frame and when i go to last frame, in this case my fog will be at constant distance will be shifting back as camera move towards the building. so whats the solution for this and which is the best and easiest way to achieve such fog?
People please do let me know if there is any tutorial or any one can help me to achieve this in after effects.
Please do help me as I dont know how to achieve this in after effects.
Thanx for your help

[Charles Black]

Isn't it normal for the fog to back up when moving forward if your in it? Or do you just want the fog to cover only the background buildings at a constant density? If so, then you could just increase the opacity of the fog at the end of the animation. All you have to do is to set a keyframe at the start, end and maybe some in between of the animation to control how thick the fog is at a certain point. You can also mess around with the levels and curves adjustments for better control over the effect.